mysql cart: simplify user env var check

Eliminate the "`-n`" test for `OPENSHIFT_MYSQL_START_TIMEOUT` (and
...`_STOP_TIMEOUT`) and replace it with the `set -u` safe
`${OPENSHIFT_MYSQL_START_TIMEOUT:-}` expansion
Merge pull request #5816 from dobbymoodge/mysql_cart_tweak

Merged by openshift-bot