Fix crash if zooming/picking on document with no pages

Traceback (most recent call last):
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.windows.plotwindow", line 665, in mousePressEvent
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.windows.plotwindow", line 607, in doPick
AttributeError: 'NoneType' object has no attribute 'widgetBoundsIterator'
Fix crash if reediting histogram dataset with blank entry

Traceback (most recent call last):
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.qtwidgets.datasetbrowser", line 487, in _edit
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.dialogs.histodata", line 294, in recreateDataset
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.dialogs.histodata", line 238, in reEditDataset
AttributeError: 'DatasetHistoGenerator' object has no attribute 'bindataset'