fix: don't normalize scoped slot with 2+ nodes

[caugner]

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:

Resolves #12102.

Note that I inlined the local variable vnode (introduced in #11963) again, because IMO it is only used in one of the condition clauses and ultimately may have caused the necessary confusion for this bug to slip through.

Props to @JaviIG for locating the source of the issue.

[caugner]

@posva I'm curious why you removed the issue reference from the title, since it's explicitly mentioned as a requirement, or is that outdated? 😄

When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

[posva]

Thanks for this I ended up adding a test and a smaller change: #12104

It seems outdated, yeah