Make sure we don't end up with an empty PYTHONPATH (#16240)

When the PYTHONPATH is an empty string python will treat it as though
the cwd is in the PYTHONPATH.  This can be undesirable.  So make sure we
delete PYTHONPATH from the environment altgether in this case.

Fixes #16195

lib/ansible/module_utils/basic.py

@@ -2038,6 +2038,8 @@ class AnsibleModule(object):
                         if not x.endswith('/ansible_modlib.zip') \
                         and not x.endswith('/debug_dir')]
             os.environ['PYTHONPATH'] = ':'.join(pypaths)
+            if not os.environ['PYTHONPATH']:
+                del os.environ['PYTHONPATH']
 
         # create a printable version of the command for use
         # in reporting later, which strips out things like