[1.9.x] Fixed #25560 -- Made empty string related_name invalid.

Thanks to Ali Lotfi for the initial report and patch
and Tim Graham for the review.

Backport of c1b6a8a98b from master

tests/invalid_models_tests/test_relative_fields.py

@@ -662,6 +662,7 @@ class RelativeFieldTests(IsolatedModelsTestCase):
             'ends_with_whitespace_%s' % whitespace,
             'with',  # a Python keyword
             'related_name\n',
+            '',
         ]
         # Python 2 crashes on non-ASCII strings.
         if six.PY3: