ISPN-4460 Map-Reduce: Mapper sometimes receives null value

Master only. See https://issues.jboss.org/browse/ISPN-4460 for more details
I agree with Will. It makes no sense the ```CacheLoader``` to return a ```null``` value. The same thing applies for the ```apply()``` method above and the ```MapReduceCacheLoaderTask.process()```.

however, if it is ```null```, you can throw an ```IllegalStateException``` to give the hint to the user to open a JIRA.