Make the returned clearTimeout conditional

+ add unit test for that as welll

src/components/text_truncate/text_truncate.test.tsx

@@ -70,6 +70,19 @@ describe('EuiTextTruncate', () => {
       expect(clearTimeoutSpy).toHaveBeenCalledTimes(2);
       expect(clearTimeoutSpy).toHaveBeenLastCalledWith(expect.any(Number));
     });
+
+    it('does not set or clear a timeout if a duration is not passed', () => {
+      const setTimeoutSpy = jest.spyOn(window, 'setTimeout');
+      const clearTimeoutSpy = jest.spyOn(window, 'clearTimeout');
+
+      const { unmount } = render(
+        <EuiTextTruncate {...props} width={0} calculationDelayMs={0} />
+      );
+
+      expect(setTimeoutSpy).not.toHaveBeenCalled();
+      unmount();
+      expect(clearTimeoutSpy).not.toHaveBeenCalled();
+    });
   });
 
   describe('resize observer', () => {

src/components/text_truncate/text_truncate.tsx

@@ -138,14 +138,10 @@ const EuiTextTruncateWithWidth: FunctionComponent<
   // If necessary, wait a tick on mount before truncating
   const [ready, setReady] = useState(!calculationDelayMs);
   useEffect(() => {
-    let timerId: NodeJS.Timeout;
     if (calculationDelayMs) {
-      timerId = setTimeout(() => setReady(true), calculationDelayMs);
+      const timerId = setTimeout(() => setReady(true), calculationDelayMs);
+      return () => clearTimeout(timerId);
     }
-
-    return () => {
-      clearTimeout(timerId);
-    };
   }, [calculationDelayMs]);
 
   // Handle exceptions where we need to override the passed props