Check for non-string exit code after `wp post edit`

Fixes bug introduced in f592cff, where quitting out of the system editor
would erase a post, because the `content_save_pre` filter would
fail to interpret system exit codes, and treat them as an empty string.

There must be a way to write a test for behavior like this, but how to
actually do that is beyond my knowledge. I did test with several
different $EDITOR settings (vi/nano/gedit), though, and this worked each
time for me.