Fix crash in picking for zero-length path collection
Merge pull request #3592 from mdboom/picking-empty

Fix crash in picking for zero-length path collection
Previous matplotlib had a bug.  As per documentation, specifying empty
string for sym argument should prevent drawing fliers for boxplots.

As an example, the following script displays fliers in the boxplot
when it shouldn't.

    import matplotlib.pyplot as plt

    data = list(range(0,100)) + [200,-200,250,-250]
    bp = plt.boxplot(data,sym='',widths=.3)
    plt.show()

Added logic to ensure empty sym string does not display fliers.
MNT : minor refactor of patch from bmistree
BUG : Merge branch bxp_fliers_no_sym into v1.4.x

This is the code from PR #3573