do not emit PASS for gdb_test_no_output test if message is empty string.

do not emit PASS for gdb_test_no_output test if message is empty string.

This patch makes gdb_test_no_output not emit a PASS for a sucessful
test when the message is the empty string. As requested, this makes
it consistent with what gdb_test does.

2010-06-03  Joel Brobecker  <brobecker@adacore.com>

        * lib/gdb.exp (gdb_test_no_output): Do not emit pass for
        successful test if message is the empty string.