at least test that nothing crashes on null

src/test/java/org/jenkinsci/plugins/prometheus/util/FlowNodesTest.java

@@ -0,0 +1,28 @@
+package org.jenkinsci.plugins.prometheus.util;
+
+import com.cloudbees.workflow.rest.external.StageNodeExt;
+import org.jenkinsci.plugins.workflow.job.WorkflowRun;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class FlowNodesTest {
+
+    @Mock
+    private WorkflowRun run;
+
+    @Test
+    void returnEmptyListOnPassingNull() {
+        when(run.getExecution()).thenReturn(null);
+        List<StageNodeExt> result = FlowNodes.getSortedStageNodes(run);
+        assertEquals(0, result.size());
+    }
+
+}