Fix Path.get_extents for empty paths.

[QuLogic]

PR Summary

Fixes #18254

PR Checklist

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

[jklymak]

This could be fine, but I'm not sure it gives the same result as master does?

[QuLogic]

master currently crashes this test, just like the linked issue.

[jklymak]

... OK, but on master, the original issue does return a finite extent, whereas I think with this change it will not.

[dopplershift]

@jklymak Did you see @QuLogic 's comment in #18254?

This works in master because we only reverted #17543 for 3.3.x. On master, there are no facecolors because 'none' is an unfilled marker, so the single path optimization is not checked, and then Path.get_extents is not called. The bug is still there, it's just not triggered.

So I think that's why master was working. Unless I'm misunderstanding something here...

[dopplershift]

Mac failures unrelated. Merging...