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

Fixes #187

Author: j00bar

django_jsonform/validators.py

@@ -299,6 +299,7 @@ def validate_string(self, schema, data, coords, raise_exc=False):
             trimmed_data = data.strip()
         else:
             trimmed_data = data
+            data = data or ""
 
         is_empty = not trimmed_data