Consistently only assign something to $clang_format if it's a good
version, and also consistently return undef if we found no good version.

Change-Id: Iadbbb56a5c15dfaeec5c80e3cc8fcc78b787c04b
Reviewed-on: https://gerrit.libreoffice.org/46489Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>