fix: ssrRef ignores null value

[wszerad]

🐛 The bug
If we set ssrRef to null it becomes undefined at client-side.

🛠️ To reproduce
it's because of nullish coalescing operator:
https://github.com/nuxt-community/composition-api/blob/main/src/runtime/composables/ssr-ref.ts#L60

🌈 Expected behaviour
Treat null as "defined" value of ssrRef