Always save out an empty .pot at minimum

[mhsdef]

LocalCI server side always looks for a localci-new-strings.pot file (https://github.com/Automattic/gp-localci/blob/master/includes/ci-adapters/circle-ci-adapter.php#L87) -- even when it is empty. I personally think that's a good behavior as it gives us some assurance the client at least ran.

Presuming we agree on that as desirable -- it means we need to always generate that file from the client even when we bail (https://github.com/Automattic/gp-localci-client/blob/master/generate-new-strings-pot.sh#L64). Otherwise we can cause false error responses at https://github.com/Automattic/gp-localci/blob/master/gp-localci.php#L65.