Fix log compression bug when no logs specified

[blimlim]

This PR closes #541.

Regex patterns for finding log files previously broke when no log files were specified, as done for CICE5. Instead of matching no files, the get_log_files() method would match every file in the model work directory.

This PR adds checks at the start of the get_log_files() and compress_log_files() methods, causing them to exit early when no log files are specified, and when no log files are found.

It also adds a unit test test_log_compression_no_logs() to test_cice.py which makes sure that the compression method does nothing when no log files are specified.

With these changes, OM2 runs without any issues.

[coveralls]

coverage: 59.003% (+0.2%) from 58.788%
when pulling cdfd675 on ACCESS-NRI:541-no-log-compression-bug
into 0c2a837 on payu-org:master.

[anton-seaice]

The update version runs OM2 successfully - I didn't test with esm1.5.

You can set

       self.logs_to_compress = [r"ice.log.task_[0-9]{4}"]
       self.log_tar_name = "ice.logfiles.tar.gz"

for cice5 if you'd like, but no particular need to.

[blimlim]

Thanks for the review @anton-seaice! I'll leave out the CICE5 log files for now, but we can always add them in later if they do cause issue. I've just done an extra test with ESM1.5 and everything's working there