fix(v-slot): fix scoped slot normalization combined with v-if

[posva]

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:
Fixes #12102
Closes #12103

[caugner]

LGTM, even though my concerns regarding the vnode variable remain (res.length == 0 vs res.length === 1 seems more intuitive to me).

[caugner]

@posva Thanks for the fix. Two questions:

1. Shouldn't this PR be added to the 2.6.14 project?
2. Should we expect a 2.6.14 release these days or rather in weeks?

[posva]

A new release in days