Fixed potential NPE in SharedEntityManagerCreator

Fix SharedEntityManagerCreator.createSharedEntityManager to correctly
forward an empty array in case that EntityManagerFactoryInfo returns
null from getEntityManagerInterface().

Previously the var-args invocation had turned null into an array with
a null value which caused a NullPointerException in the call to
Proxy.newProxyInstance() further down the call stack.

Issue: SPR-10678
Merge pull request #303 from olivergierke/SPR-10678

# By Oliver Gierke
* SPR-10678:
  Fixed potential NPE in SharedEntityManagerCreator