Return None for any missing BaseModel attribute

Accessing a missing attribute in BaseModel raises an AttributeError, but
as this happens quite frequently with objects that otherwise contain
valid data, it should not be enough to cause the API query to fail.
Instead, we can safely return None in place of a missing attribute.

Author: gnofs

src/cbapi/oldmodels.py

@@ -141,10 +141,7 @@ def _attribute(self, attrname, default=None):
         if attrname in self._info:
             return self._info[attrname]
 
-        if default is not None:
-            return default
-
-        raise AttributeError()
+        return default
 
     def get(self, attrname, default_val=None):
         try: