Fix warning in data merge strategy

[posva]

This allows to reuse the data merge strategy without forcing it to be
a function when used in a child

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:

[yyx990803]

I don't think this would be sufficient. The code below this condition check assumes childVal is a function.

[posva]

That's true. I'll have a deeper look whenever I can.

[yyx990803]

Seems to be covered in #6025