Issue 1:
Bug report

Bug summary

calling scatter with marker="None" results in ValueError.

Code for reproduction

# Paste your code here
#
#
from matplotlib import pyplot as plt
plt.scatter([1], [1], marker="None")

Actual outcome

# If applicable, paste the console output here
#
#
Traceback (most recent call last):
File "/usr/lib/python3.7/tkinter/__init__.py", line 1705, in __call__
return self.func(*args)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/backends/_backend_tk.py", line 242, in resize
self.draw()
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/backends/backend_tkagg.py", line 9, in draw
super(FigureCanvasTkAgg, self).draw()
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py", line 407, in draw
self.figure.draw(self.renderer)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/figure.py", line 1864, in draw
renderer, self, artists, self.suppressComposite)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/image.py", line 131, in _draw_list_compositing_images
a.draw(renderer)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/cbook/deprecation.py", line 411, in wrapper
return func(*inner_args, **inner_kwargs)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/axes/_base.py", line 2748, in draw
mimage._draw_list_compositing_images(renderer, self, artists)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/image.py", line 131, in _draw_list_compositing_images
a.draw(renderer)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/collections.py", line 931, in draw
Collection.draw(self, renderer)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/collections.py", line 385, in draw
extents = paths[0].get_extents(combined_transform)
File "path_to_venv/env/lib/python3.7/site-packages/matplotlib/path.py", line 603, in get_extents
return Bbox([xys.min(axis=0), xys.max(axis=0)])
File "path_to_venv/env/lib/python3.7/site-packages/numpy/core/_methods.py", line 43, in _amin
return umr_minimum(a, axis, None, out, keepdims, initial, where)
ValueError: zero-size array to reduction operation minimum which has no identity

Expected outcome

Blank plot. Same as if alpha=0 was specified.

Matplotlib version

Operating system: Ubuntu 18.04.4 LTS

Matplotlib version: 3.3.1

Matplotlib backend (print(matplotlib.get_backend())): TkAgg

Python version: 3.7.5

Jupyter version (if applicable):

Other libraries:

Installed with pip

Issue 2:
Bug report

Bug summary

plt.scatter() raises upon drawing when called with marker='' (no errors with 3.3.0).

This change of behavior broke seaborn.

Code for reproduction

import matplotlib.pyplot as plt
plt.scatter([], [], marker='') # fails also with plt.scatter([1,2,3], [1,2,3], marker='')
plt.show()

Actual outcome

Traceback (most recent call last):
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/backends/backend_macosx.py", line 61, in _draw
self.figure.draw(renderer)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/figure.py", line 1863, in draw
mimage._draw_list_compositing_images(
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/image.py", line 131, in _draw_list_compositing_images
a.draw(renderer)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/cbook/deprecation.py", line 411, in wrapper
return func(*inner_args, **inner_kwargs)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 2748, in draw
mimage._draw_list_compositing_images(renderer, self, artists)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/image.py", line 131, in _draw_list_compositing_images
a.draw(renderer)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/collections.py", line 931, in draw
Collection.draw(self, renderer)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/artist.py", line 41, in draw_wrapper
return draw(artist, renderer, *args, **kwargs)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/collections.py", line 385, in draw
extents = paths[0].get_extents(combined_transform)
File "/Users/maoz/.local/lib/python3.8/site-packages/matplotlib/path.py", line 603, in get_extents
return Bbox([xys.min(axis=0), xys.max(axis=0)])
File "/Users/maoz/miniconda/envs/seaborn_devenv/lib/python3.8/site-packages/numpy/core/_methods.py", line 43, in _amin
return umr_minimum(a, axis, None, out, keepdims, initial, where)
ValueError: zero-size array to reduction operation minimum which has no identity

Expected outcome

Not to fail.

Matplotlib version

Operating system: MacOS

Matplotlib version: 3.3.1

Matplotlib backend (print(matplotlib.get_backend())): MacOSX (also reproduces with %inline)

Python version: 3.8.3

Jupyter version (if applicable):

Other libraries: numpy 1.19.1

matplotlib installed through pip