ASACORE-917 fix Java signal handler registration

Apparently, the JNI code passes empty string instead of NULL when
calling BusObject::RegisterSignalHandler() without source path
qualifier.

Change-Id: Icde36ebe4a78701b821ce4aa064ac0a76a0e4cde
Signed-off-by: Meggan Ehret <meggan@qeo.org>