[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 c1b6a8a from master

django/db/models/fields/related.py

@@ -124,7 +124,7 @@ def _check_related_name_is_valid(self):
         import re
         import keyword
         related_name = self.remote_field.related_name
-        if not related_name:
+        if related_name is None:
             return []
         is_valid_id = True
         if keyword.iskeyword(related_name):

django/db/models/fields/reverse_related.py

@@ -154,7 +154,7 @@ def get_db_prep_lookup(self, lookup_type, value, connection, prepared=False):
 
     def is_hidden(self):
         "Should the related object be hidden?"
-        return self.related_name is not None and self.related_name[-1] == '+'
+        return bool(self.related_name) and self.related_name[-1] == '+'
 
     def get_joining_columns(self):
         return self.field.get_reverse_joining_columns()

tests/invalid_models_tests/test_relative_fields.py

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