ci/gitlab_gql: make sure the job exists before checking its dependencies

Some jobs may depend on jobs that are not found in all pipelines, so let's first check that the job exists before trying to get its dependencies.

Fixes: 278fc1c2 ("ci/bin: gql: Improve queries for jobs/stages retrieval") Signed-off-by: Martin Roukala (né Peres) martin.roukala@mupuf.org