types: improve isDef typing (#7124)

* types: improve isDef typing

* chore: update

src/utils/index.ts

@@ -9,7 +9,7 @@ export const isServer: boolean = Vue.prototype.$isServer;
 // eslint-disable-next-line @typescript-eslint/no-empty-function
 export function noop() {}
 
-export function isDef(val: unknown): boolean {
+export function isDef<T>(val: T): val is NonNullable<T> {
   return val !== undefined && val !== null;
 }