BF: ignore CLOSEPOLY after NaN in PathNanRemover

[brunobeltran]

PR Summary

Fixes #17868.

In short, the current PathNanRemover always keeps CLOSEPOLY "vertices" around, even if they contain NaN. This is arguably okay (since it is documented in comments that the values of CLOSEPOLY vertices are ignored). However, because of this, if a CLOSEPOLY occurs before any non-NaN segments of a Path are encountered, then PathNanRemover (and, by proxy, Path.cleaned(remove_nans=True) will return a malformed Path that starts with a CLOSEPOLY.

Current behavior:

>>> p = Path(
    [[np.nan, np.nan], [np.nan, np.nan]],
    [1, 79])
>>> p.cleaned(remove_nans=True)
Path(array([[np.nan, np.nan], [0., 0.]]), array([79, 0]))

New behavior

>>> p.cleaned(remove_nans=True)
Path(array([[0., 0.]]), array([0], dtype=uint8))

PR Checklist

• Has Pytest style unit tests
• Code is Flake 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/next_api_changes/* if API changed in a backward-incompatible way

[QuLogic]

I think we ignore the point entirely for CLOSEPOLY, but what happens for something like?

p = Path(
    [[0, 0], [np.nan, np.nan]],
    [1, 79])

[brunobeltran]

what happens for something like?

p = Path(
    [[0, 0], [np.nan, np.nan]],
    [1, 79])

Great question. It leaves it as-is. (Same as previous behavior).

>>> p.cleaned(remove_nans=True)
Path(array([[ 0.,  0.],
       [nan, nan],
       [ 0.,  0.]]), array([ 1, 79,  0], dtype=uint8))

I think this makes sense because

1. We document that the vertex attached to the CLOSEPOLY command is ignored, and
2. We were already leaving all CLOSEPOLY's in before, including these NaN ones

So this PR only changes the behavior when leaving a CLOSEPOLY in would result in an ill-defined Path.

[brunobeltran]

I'm aware that this doesn't play well with PathSimplifier:

>>> p.cleaned(remove_nans=True, simplify=True)
Path(array([[ 0.,  0.],
       [nan, nan],
       [nan, nan],
       [ 0.,  0.]]), array([1, 2, 2, 0], dtype=uint8))

but this is also existing behavior, and I haven't taken the time to grok the full PathSimplifer algorithm yet to know whether or not it's easy to do even better.

[brunobeltran]

Per the call, added tests that check the "optimized" code path (i.e. empty codes array) and explicitly added the breaking example from #17868 as a test case.

[tacaswell]

Still approve.

[QuLogic]

I pushed a small whitespace fix.