Add explicit undefined typing to consoleWarnSpy

Author: ernst-dev

src/internal/stable-callback/__tests__/use-stable-event-handler.test.tsx

@@ -33,7 +33,7 @@ const Component = React.forwardRef(({ onChange }: Props, ref: React.Ref<Ref>) =>
   return null;
 });
 
-let consoleWarnSpy: jest.SpyInstance;
+let consoleWarnSpy: jest.SpyInstance | undefined;
 afterEach(() => {
   consoleWarnSpy?.mockRestore();
 });