CLIMATE-797 Fix attribute error when dataset has no mask attribute
CLIMATE-797 Fix attribute error when dataset has no mask attribute

Omkar20895/climate,Omkar20895/climate,lewismc/climate,Omkar20895/climate,agoodm/climate,apache/climate,lewismc/climate,agoodm/climate,Omkar20895/climate,apache/climate,agoodm/climate,apache/climate,apache/climate,apache/climate,agoodm/climate,lewismc/climate,agoodm/climate,lewismc/climate,Omkar20895/climate,lewismc/climate