cyclades: Fix bug in SeparatedValuesField

Return [] instead of None, if the field is null.

snf-cyclades-app/synnefo/db/fields.py

@@ -42,7 +42,7 @@ class SeparatedValuesField(models.TextField):
 
     def to_python(self, value):
         if not value:
-            return
+            return []
         if isinstance(value, list):
             return value
         return value.split(self.delimiter)