ISPN-2609 Infinispan SpringCache throws java.lang.NullPointerException:
Null values are not supported!

Add support for storing null values in and retrieving null values from a
SpringCache. Use solution provided by Roland Csupor as a starting point.

* Introduce NullValue, a singleton class that represents a null value in a
cache.
* Make SpringCache use NullValue for representing null values in wrapped
Infinispan cache.