stdlib.python_version: cache no python too

This is important for HPC or any computer without Python, to not
waste time rechecking

Author: scivision

+stdlib/python_version.m

@@ -1,6 +1,8 @@
 %% PYTHON_VERSION get the Python version used by MATLAB
 %
-% uses persistent variable to cache the Python version
+% uses persistent variable to cache the Python version.
+% If the environment changes, the cached version will be invalid.
+% this cache is cleared by "clear stdlib.python_version"
 %
 %%% Inputs
 % * force_old: (optional) boolean flag to force checking of Python on Matlab < R2022a
@@ -43,9 +45,7 @@
   msg = e.message;
 end
 
-% cache the result
-if ~isempty(v)
-  stdlib_py_version = v;
-end
+% cache the result - even if empty -- because the check takes up to 1000 ms say on HPC
+stdlib_py_version = v;
 
 end