Updating Associated Metadata Provider to follow the existing pattern.

cc @dougbu @rynowak 
Since we merge the include properties at the type and parameter using a .Intersect, if there is a non empty collection of included properties at type and/or parameter level and there are no common properties, it would return an empty collection. If we did not remove the check of null or empty it would have caused the inclusion of all properties where there should have been no property that should have been included. 
BTW this is temporary since with the BindAttribute changes #1546, we would be moving to a predicate based model (even in model metadata) entirely.