fix(ci): guard the matrix deployment jobs against an empty job list

wait-for-deployment builds its matrix from
build-helmreleases.outputs.staging_helmreleases, and
create-auto-merge-prod-pr from updated_prod_cluster_names. Neither checked
the value for '[]' first, unlike every other matrix job in this file
(services-build and helm-charts-build both guard on != '[]' && != '').

A matrix that expands to zero combinations cannot produce a job, so the
reusable-workflow call reports failure. On PR #2092 that surfaced as a
gate failure with no failing job anywhere: 15 of the pipeline's 16 jobs
were success or skipped, wait-for-deployment had no check run at all, and
alls-green received gooddata-ruby-lcm-promote-pipeline result=failure with
empty outputs.

The trigger was #2094. Before it, create-staging-pr failed outright when
there was nothing to deploy, so wait-for-deployment's
!contains(needs.*.result, 'failure') was false and the job was skipped
before its matrix was ever evaluated -- it showed up as 'skipped' in run
32728855771. Once create-staging-pr correctly succeeded on a no-op, the
condition passed, the matrix was evaluated for the first time in this
state, and it was empty. #2094 did not introduce this, it removed the
failure that was masking it.

staging_helmreleases is empty whenever stg1 already runs the built chart,
which is the normal state after a previous attempt delivered its staging
PR -- gitops-deployments#26557 in this case.

create-auto-merge-prod-pr carries the same defect. It is latent here
because the gate sets AUTO_MERGE: false, but it is live for any consumer
that auto-merges into an already up-to-date production cluster.

JIRA: GRIF-518

Author: milandufek

.github/workflows/lcm-pipeline.yaml

@@ -225,6 +225,8 @@ jobs:
       !cancelled() &&
       !contains(needs.*.result, 'failure') &&
       needs.build-helmreleases.result == 'success' &&
+      needs.build-helmreleases.outputs.staging_helmreleases != '[]' &&
+      needs.build-helmreleases.outputs.staging_helmreleases != '' &&
       inputs.deploy
     strategy:
       matrix:
@@ -335,6 +337,8 @@ jobs:
       !cancelled() &&
       !contains(needs.*.result, 'failure') &&
       needs.build-helmreleases.result == 'success' &&
+      needs.build-helmreleases.outputs.updated_prod_cluster_names != '[]' &&
+      needs.build-helmreleases.outputs.updated_prod_cluster_names != '' &&
       inputs.AUTO_MERGE &&
       inputs.deploy
     strategy: