[COLLECTIONS-536] Improved null check. Thanks to Tagir Valeev.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1635303 13f79535-47bb-0310-9956-ffa450edef68
[COLLECTIONS-518] Make AbstractIterableGetMapDecorator abstract. Thanks to Dipanjan Laha.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1635305 13f79535-47bb-0310-9956-ffa450edef68