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

---------

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

Author: 3 people

tools/pybind11NewTools.cmake

@@ -110,15 +110,17 @@ if(NOT DEFINED ${_Python}_EXECUTABLE)
 
 endif()
 
-if(NOT ${_Python}_EXECUTABLE STREQUAL PYBIND11_PYTHON_EXECUTABLE_LAST)
+if(DEFINED PYBIND11_PYTHON_EXECUTABLE_LAST AND NOT ${_Python}_EXECUTABLE STREQUAL
+                                               PYBIND11_PYTHON_EXECUTABLE_LAST)
   # Detect changes to the Python version/binary in subsequent CMake runs, and refresh config if needed
   unset(PYTHON_IS_DEBUG CACHE)
   unset(PYTHON_MODULE_EXTENSION CACHE)
-  set(PYBIND11_PYTHON_EXECUTABLE_LAST
-      "${${_Python}_EXECUTABLE}"
-      CACHE INTERNAL "Python executable during the last CMake run")
 endif()
 
+set(PYBIND11_PYTHON_EXECUTABLE_LAST
+    "${${_Python}_EXECUTABLE}"
+    CACHE INTERNAL "Python executable during the last CMake run")
+
 if(NOT DEFINED PYTHON_IS_DEBUG)
   # Debug check - see https://stackoverflow.com/questions/646518/python-how-to-detect-debug-Interpreter
   execute_process(