Make the JavaHL ByteArray wrapper more consistent with the String wrapper.

* subversion/bindings/javahl/native/jniwrapper/jni_array.hpp:
  (ByteArray::ByteArray): Initialize the length of a null array to 0.
  (ByteArray::Contents::Contents): Do not get the elements of a null array.
  (ByteArray::Contents::get_string): Protect against null contents.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540483 13f79535-47bb-0310-9956-ffa450edef68