NPE fixed in RiakMap getDatatype functions

HashMap used to store the datatypes mapping in RiakMap,  returns null when there is no mapping for the given  key.  http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html#get(java.lang.Object)

This patch corrects the Null pointer exception thrown when querying the RiakMap for getMap(), getCounter() etc API calls and returns a null as specified in the javadoc.