Fixed a bug JavaHL stream translation utilility, where keywords were
not contracted as expected when given a keywords map with null values.

[in subversion/bindings/javahl]
* native/org_apache_subversion_javahl_util_SubstLib.cpp
  (KeywordHashBuilder::m_empty): New member; an empty svn_string_t.
  (KeywordHashBuilder::KeywordHashBuilder): Initialize m_empty.
  (KeywordHashBuilder::operator()): Insert an empty string into
   the keywords hash if the value in the map was null. Before this
   change, a NULL string would be inserted, which apr_hash_map
   interprets as a deletion.

* tests/org/apache/subversion/javahl/UtilTests.java
  (UtilTests.testTranslateStream): Test keyword contraction with
   null values in the keywords map, and irrelevant values for the
   keywords builder parameters in the short-circuit overloads.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545205 13f79535-47bb-0310-9956-ffa450edef68