Fixed a thinko in the JavaHL implementation of the Ev2 driver.

* subversion/bindings/javahl/native/CommitEditor.cpp
  (CommitEditor::alterDirectory, CommitEditor::alterFile): An empty
   property set must be passed on as an empty hash, not a NULL hash,
   since the latter means the props should be left alone.

* subversion/bindings/javahl/tests/org/apache/subversion/javahl/SVNRemoteTests.java
  (SVNRemoteTests.testEditorDeleteFileProps): New test case for
   remote property deletion.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537542 13f79535-47bb-0310-9956-ffa450edef68