Remove some maybe-uninitialized warnings (#5516)

* Remove some maybe-uninitialized warnings

In the Eigen matrix type_caster, resize the matrix instead of assigning with a new one when the matrix size needs to be adjusted.

This can remove lots of compiling warnings about "maybe-uninitialized".

* Revert "Remove some maybe-uninitialized warnings"

This reverts commit 7d5a9b41aaf6878f9ff27aa0fb68c6de4a409a01.

* Suppress `-Wmaybe-uninitialized` warning

Reproducer: https://github.com/pybind/pybind11/pull/5516#issuecomment-2645846295

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Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>