Fix build for ports that have a LICENSE if DISABLE_LICENSES is defined.

The behavior of the ask-license target should be to silently succeed instead
of returning an error, IMHO, but I can test for this condition and skip the
ask-license phase in portmaster, too.

While here, I'm slightly simplifying the fix implemented in r460294.

PR: 225699
Submitted by: dewayne@heuristicsystems.com.au
Approved by: antoine (implicit)