fix(types): make $refs undefined possible

[WORMSS]

Added undefined as a return type of $refs as it was quite hard to convince typescript that something COULD be undefined if you type in a ref that doesn't exist,
I also changed the two array statements from Element[] | Vue[] to (Element | Vue)[] because it's not guaranteed that they are ALL Elements or they are ALL Vue components. (Very likely, yes, but not always)
For example

<template v-for="(a, index) of b">
  <div v-if="index % 2 === 0" ref="test"></div>
  <my-component v-else ref="test"></my-component>
</template>

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: Typings

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: