fix: #5064 Handle case when line has only one point

cypress/integration/rendering/flowchart-v2.spec.js

@@ -729,6 +729,18 @@ A ~~~ B
       {}
     );
   });
+
+  it('5064: Should render when subgraph child has links to outside node and subgraph', () => {
+    imgSnapshotTest(
+      `flowchart TB
+    Out --> In
+    subgraph Sub
+      In
+    end
+    Sub --> In`
+    );
+  });
+
   describe('Markdown strings flowchart (#4220)', () => {
     describe('html labels', () => {
       it('With styling and classes', () => {

packages/mermaid/src/utils/lineWithOffset.ts

@@ -24,6 +24,9 @@ function calculateDeltaAndAngle(
 ): { angle: number; deltaX: number; deltaY: number } {
   point1 = pointTransformer(point1);
   point2 = pointTransformer(point2);
+  if (point1 === undefined || point2 === undefined) {
+    return { angle: 0, deltaX: 0, deltaY: 0 };
+  }
   const [x1, y1] = [point1.x, point1.y];
   const [x2, y2] = [point2.x, point2.y];
   const deltaX = x2 - x1;