fix(runtime-core): update initial old value to deep watch callback in ssr …

[ioslh]

Currently watch usage below in SSR context will cause iterate error.

setup(){
  const a = ref(1)
  const b = ref(2)
  watch([a, b], ([newA, newB], [oldA, oldB]) => {
    // ...
  }, { deep: true, immediate: true })
}

because second parameter for the first time calling of watch callback is undefined.
In this PR, we change it to [] for multi-source, make code work.