Fix SAMLIdentityProvider backend filling user details with None.

It's a general convention that this project follows to
use '' instead of `None`. Most of of the other backends use ''.

social_core/backends/saml.py

@@ -79,7 +79,7 @@ def get_attr(self, attributes, conf_key, default_attribute):
             if len(value):
                 value = value[0]
             else:
-                value = None
+                value = ''
         return value
 
     @property