Makefile: Handle unexpected empty var. values

Fixes #14021

Substitution values built from `$(shell ...)` output can easily be empty
due to the shell's default `pipefail` behavior.  This can also hide
non-zero exit codes, similarly resulting in empty values being set.
While not a perfect fix, the situation is improved by using the
`err_if_empty` function in all cases where empty values would be
unexpected.  Remove the definitions for `GIT_BRANCH` and
`GIT_BRANCH_CLEAN` which don't seem to actually be used anywhere
(including in code).

Add a simple release-test to verify `podman info` outputs a non-empty
value for "GitCommit".

Signed-off-by: Chris Evich <cevich@redhat.com>

contrib/cirrus/runner.sh

@@ -312,6 +312,11 @@ function _run_release() {
     if [[ -n "$dev" ]]; then
         die "Releases must never contain '-dev' in output of 'podman info' ($dev)"
     fi
+
+    commit=$(bin/podman info --format='{{.Version.GitCommit}}' | tr -d '[:space:]')
+    if [[ -z "$commit" ]]; then
+        die "Releases must contain a non-empty Version.GitCommit in 'podman info'"
+    fi
     msg "All OK"
 }