fix @grafana/flamegraph types

packages/grafana-flamegraph/src/FlameGraph/FlameGraph.test.tsx

@@ -16,7 +16,7 @@ jest.mock('react-use', () => {
   return {
     ...reactUse,
     useMeasure: () => {
-      const ref = React.useRef();
+      const ref = React.useRef(null);
       return [ref, { width: 1600 }];
     },
   };

packages/grafana-flamegraph/src/FlameGraphContainer.test.tsx

@@ -21,7 +21,7 @@ jest.mock('@grafana/assistant', () => ({
 jest.mock('react-use', () => ({
   ...jest.requireActual('react-use'),
   useMeasure: () => {
-    const ref = useRef();
+    const ref = useRef(null);
     return [ref, { width: 1600 }];
   },
 }));