Null values for not-required strings are treated as empty strings

Fixes #187

Author: j00bar

django_jsonform/validators.py

@@ -298,7 +298,7 @@ def validate_string(self, schema, data, coords, raise_exc=False):
         if isinstance(data, str):
             trimmed_data = data.strip()
         else:
-            trimmed_data = data
+            trimmed_data = data or ""
 
         is_empty = not trimmed_data