chore(ci): run all system-tests scenario even if empty when running on master/sc… (#8067)

* Run all system-tests scenario even if empty when running on master/schedule

* ci: gate skip_empty_scenarios on ref instead of event_name

Using github.event_name != 'push' also disables scenario skipping for
merge-queue pushes (mq-working-branch-master-*). Gate on
github.ref != 'refs/heads/master' to only run all scenarios on master.

Author: nccatoni

.github/workflows/system-tests.yml

@@ -61,5 +61,5 @@ jobs:
       scenarios_groups: tracer-release
       excluded_scenarios: APM_TRACING_E2E_OTEL,APM_TRACING_E2E_SINGLE_SPAN # require AWS and datadog credentials
       parametric_job_count: 8
-      skip_empty_scenarios: true
+      skip_empty_scenarios: ${{ github.ref != 'refs/heads/master' && github.event_name != 'schedule' }}
       push_to_test_optimization: ${{ github.actor != 'dependabot[bot]' }}