Fix Poly3DCollection ragged padding overflow warnings - #32310

PR summary

When Poly3DCollection gets polygons with different vertex counts, _get_vector packs them into a rectangular array with np.empty(). Unused padding is marked in _invalid_vertices, but do_3d_projection still projects the whole array first. Uninitialized padding can hold huge float values and trigger nondeterministic RuntimeWarning: overflow encountered in dot.

This change initializes that padded storage with np.nan instead. NaN values do not overflow in the projection math, and do_3d_projection already ignores invalid floating point results. A regression test builds a square plus triangle collection, checks that padding is NaN, and draws under np.errstate(over="raise").

Closes #32272

Note: #32279 proposed the same fix earlier and was closed because of the automated-contribution account policy, not because the change itself was wrong.

AI Disclosure

I used Cursor to help locate the np.empty padding path, draft the regression test, and prepare the PR text. I reviewed the code path in Poly3DCollection._get_vector / do_3d_projection, confirmed why #32279 was closed, and verified the overflow reproduction plus the NaN-padding behavior locally before opening this PR.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• New and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
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• [N/A] Documentation complies with general and docstring guidelines