ci: work around old records of GitHub runs

ci: work around old records of GitHub runs

Apparently older GitHub runs at least _sometimes_ lack information about
the `head_commit` (and therefore the `ci-config` check will fail with
"TypeError: Cannot read property 'tree_id' of null") in the check added
in 7d78d5fc1a9 (ci: skip GitHub workflow runs for already-tested
commits/trees, 2020-10-08).

Let's work around this by adding a defensive condition.

Reported-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>