feat(props): allow defining a required prop as null

[posva]

Based on #1961. It wasn't possible to specify a required prop of type object and pass null as the
value

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

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

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:

The tests cases reflect what I'm trying to fix. I remember this was pointed out multiple times in the past. We cannot explicitely pass a null prop when the type is Object and the prop is required. This would keep current behaviour but allow having the type null for explicit null values. I want to add more tests and maybe refactor but I remember this was problematic as it could break some validation, so I would like so feedback about types you may use that would break with this

[Justineo]

Some potential inconsistency here:

props: {
  foo: null, // skip
  bar: {
    type: null // skip
  },
  baz: {
    type: [String, null] // String or null
  }
}

While using constructors, all three usage share the same semantics.

[posva]

Added some tests to reflect that as well

[dilraj-vidyard]

There are tests for this now. Is there still something preventing this from going in?