test: Fix git hash emptiness check in postgres-head Dockerfile

[sehrope]

Fixes emptiness check in postgres-head building Dockerfile. Had a bash-ism so it was always failing the emptiness check but didn't notice locally as as the docker/bin/postgres-head script always supplies a value. Updated check works in my fork: https://github.com/sehrope/pgjdbc/runs/2676337730?check_suite_focus=true#step:6:18