Fix thinko in handling empty copy source array in JavaHL.

[in subversion/bindings/javahl]
* src/org/apache/subversion/javahl/ISVNClient.java (ISVNClient.copy):
   Document that a NPE is thrown if the list of sources is empty.
* native/SVNClient.cpp (SVNClient::copy):
   Throw a NullPointerException instead of using an invalid error code.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543325 13f79535-47bb-0310-9956-ffa450edef68