Skip journal corruption check if no P4JOURNAL exists.
If the P4JOURNAL file doesn't exist for whatever reason, the
code does a 'tail' on a non-existent file, resutling in a
harmless error in the p4d_init.log file.
Changed wording from "Preflight journal corrpution check" to
"Preflight journal health check", as it just feels wrong to have
the words "journal corruption" appear every time p4d starts. And
in at least one case I saw a customer have terminal auto-coloring
react, highlighthing in red and bolding the word "corruption,"
making it even scarier, even though the test passed.
Fixed an unrelated shellcheck issue (no functional effect) to
maintain shellcheck compliance.