Don't log empty test result messages

Don't log empty test result messages

The messages obtained from test results may contain stuff like detailed
failure/error information, tracebacks etc. In many cases, the message
will be empty, in which case it doesn't need to be logged.

For an example, run test with many test cases, e.g.
verify/test_verify.py, and inspect the tail of the pytest.log.txt before
and after this commit.

(cherry picked from commit 0a6b0cf68c6553f7eeef06fb507c1b28b9c61f38)