fix(cmake): skip empty PYBIND11_PYTHON_EXECUTABLE_LAST for the first cmake run (#4856)

* fix(cmake): skip empty PYBIND11_PYTHON_EXECUTABLE_LAST for the first cmake run

* style: pre-commit fixes

* Update pybind11NewTools.cmake

* style: pre-commit fixes

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>