[3.0.x] Refs #31040 -- Fixed crypt.crypt() call in test_hashers.py.

An empty string is invalid salt in Python 3 and raises exception since
Python 3.9, see https://bugs.python.org/issue38402.
Backport of 1960d55f8b from master

tests/auth_tests/test_hashers.py

@@ -16,7 +16,7 @@ except ImportError:
     crypt = None
 else:
     # On some platforms (e.g. OpenBSD), crypt.crypt() always return None.
-    if crypt.crypt('', '') is None:
+    if crypt.crypt('') is None:
         crypt = None
 
 try: