tests/basics/string_join.py: Add test case where argument is not iterable.

tests/basics/string_join.py

@@ -13,6 +13,11 @@ print(','.join('abc' for i in range(5)))
 
 print(b','.join([b'abc', b'123']))
 
+try:
+    ''.join(None)
+except TypeError:
+    print("TypeError")
+
 try:
     print(b','.join(['abc', b'123']))
 except TypeError: