Added explicit null return to completeUnitOfWork() to resolve Flow error (#9299)

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -684,6 +684,10 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         return null;
       }
     }
+
+    // Without this explicit null return Flow complains of invalid return type
+    // TODO Remove the above while(true) loop
+    return null;
   }
 
   function performUnitOfWork(workInProgress: Fiber): Fiber | null {