FIX: Postscript allow empty markers

[jklymak]

PR Summary

If you set a marker to have markerfacecolor='none' with the eps backend, no marker is drawn, even if the edgecolor is non-zero. Thats silly. This checks and if none, just doesn't issue the fill command instead of returning before anything is drawn...

I couldn't figure out how to make a postscript test. The saved image is in pdf, but the test spits out an eps, so...

Closes #10921

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[dstansby]

I reckon this should go into 2.x since it's a bugfix

[jklymak]

Hmmm did we really want to backport this? I dont mind but I think we were only backportibg regressions and crashes.

[jkseppan]

The documentation says "Everything else (regressions against 1.x versions, bugs/api inconsistencies the user can work around in their code) are on a case-by-case basis, should be low-risk, and need someone to advocate for and shepherd through the backport." I think this one is low-risk and I read @dstansby's comment as being willing to do the shepherding.