gitlab-ci: fix testing whether a variable with a given name is set or not

The previous code considered unset variables the same as set-but-empty;
sometimes setting a variable as something empty is meaningful, so let's
pass them through properly.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6389>
1 file changed