astakos: Fix migration model definition

Migration 0081 had not incorporated the changes of the previous migrations,
and thus still considered AstakosUser.base_project as non null.

snf-astakos-app/astakos/im/migrations/0081_auto__add_field_astakosuserauthprovider_last_login_at.py

@@ -77,7 +77,7 @@ class Migration(SchemaMigration):
             'auth_token': ('django.db.models.fields.CharField', [], {'max_length': '64', 'unique': 'True', 'null': 'True', 'blank': 'True'}),
             'auth_token_created': ('django.db.models.fields.DateTimeField', [], {'null': 'True'}),
             'auth_token_expires': ('django.db.models.fields.DateTimeField', [], {'null': 'True'}),
-            'base_project': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'base_user'", 'to': "orm['im.Project']"}),
+            'base_project': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'base_user'", 'null': 'True', 'to': "orm['im.Project']"}),
             'date_signed_terms': ('django.db.models.fields.DateTimeField', [], {'null': 'True', 'blank': 'True'}),
             'deactivated_at': ('django.db.models.fields.DateTimeField', [], {'null': 'True', 'blank': 'True'}),
             'deactivated_reason': ('django.db.models.fields.TextField', [], {'default': 'None', 'null': 'True'}),
@@ -330,4 +330,4 @@ class Migration(SchemaMigration):
         }
     }
 
-    complete_apps = ['im']
\ No newline at end of file
+    complete_apps = ['im']