fix(runtime-core): optimize judgment

[BIMiracle]

When I use nuxt3 with pageTransition and there is await on the page, changing files triggers HMR to report an error. The leavingVNode.el value is not judged to be null, which can be seen in my demo.

Uncaught (in promise) TypeError: Cannot read properties of null (reading 'Symbol(_leaveCb)')

https://github.com/vuejs/router/issues/341