Work around a problem identified by BuildHive
Apparently the signal handling is not quite correct in the fsckobject
handling (most likely we rely on a side effect that lets us still output
some message after receiving a signal 13 but in the BuildHive setup this
fails intermittently).
As a consequence, the push in t5504 does fail as expected, but fails to
output anything (unexpected). Since this is good enough for now, let's
handle an empty output as success, too.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>