Fix Faker string type assertion to properly handle empty string and None values

Author: chenyunliang520

tests/fix_faker.py

@@ -320,6 +320,11 @@ def match_any(self, spec, got, want):
                 assert got in (None, b"", bytearray(b""), memoryview(b""))
             else:
                 assert got == want
+        elif spec == str:
+            if want == '' or want is None:
+                assert got == '' or got is None
+            else:
+                assert got == want
         else:
             assert got == want