fix(runtime-core): fix immediate callback not triggered bug for multisource

[ioslh]

Fix this bug:

1. when watching multi-source with { immediate: true } option
2. and the init value of the ref value is undefined

the callback won't get called immediately as expected.

See reproduce buggy code detailed below( or sfc playground link):

// important to reproduce: set init value to Ref<undefined>
const msg = ref()

watch(msg, () => {
  console.log('this will be called immediately as expected')
}, { immediate: true })
  
watch([msg], () => {
  console.log("but this won't")
}, { immediate: true })

When determining if callback should be called immediately for the first time ( relative code here ) , the hasChanged get false for multi source(newValue is[undefined] and oldValue is [], [undefined][0]===[][0] ) while get true for single source, which cause different behaviors.

This PR fix it by compare the reference valueINITIAL_WATCHER_VALUE to newValue for multi source, just like what single source case do.
And when we call callback, pass undefined for single source and [] for multi source for the first time, keep this correct behavior.

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[LinusBorg]

I also took a stab at this in #5033