test: automation to cover nill resource selector

The auto covered upstream issue
https://github.com/prometheus-operator/prometheus-operator/issue/6018

Author: lihongyan1

test/e2e/monitoring_stack_controller_test.go

@@ -66,6 +66,9 @@ func TestMonitoringStackController(t *testing.T) {
 	}, {
 		name:     "resource selector nil propagates to Prometheus",
 		scenario: nilResrouceSelectorPropagatesToPrometheus,
+	}, {
+		name:     "prometheus with nil resource selector becomes ready",
+		scenario: nilResourceSelectorPrometheusBecomesReady,
 	}, {
 		name:     "stack spec are reflected in Prometheus",
 		scenario: reconcileStack,
@@ -174,6 +177,21 @@ func nilResrouceSelectorPropagatesToPrometheus(t *testing.T) {
 	}
 }
 
+func nilResourceSelectorPrometheusBecomesReady(t *testing.T) {
+	ms := newMonitoringStack(t, "nil-selector-ready")
+	ms.Spec.ResourceSelector = nil
+
+	err := f.K8sClient.Create(context.Background(), ms)
+	assert.NilError(t, err, "failed to create monitoring stack with nil resourceSelector")
+
+	// Verify Prometheus CR is created
+	f.AssertResourceEventuallyExists(ms.Name, ms.Namespace, &monv1.Prometheus{})(t)
+
+	// Verify Prometheus pods become ready despite nil resourceSelector
+	// covers upstream issue #932
+	f.AssertStatefulsetReady("prometheus-"+ms.Name, ms.Namespace, framework.WithTimeout(5*time.Minute))(t)
+}
+
 func promConfigDefaultsAreApplied(t *testing.T) {
 	tests := []struct {
 		name     string