BUG : don't assign color to flier props if None

Close #3336
BUG : guard marker from sym parsing in bxp

add a guard against `None` returned as `marker`
by `_process_plot_format(sym)`.
TST : added test for bxp

Adds test for case where `sym` kwarg to `bxp` does not
specify color
Merge pull request #3337 from tacaswell/bxp_flier_colors

BUG : don't assign color or marker to flier props if None