fix #6284: avoid iterating non-observable arrays

[fenduru]

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

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

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

• Yes
• No

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

While this introduces a slight behavior change, I believe the only scenario in which this would cause breakage is if an application has a non-reactive object inside of a non-reactive array and is relying on the reactivity system to detect changes to the elements in that array. This seems incredibly unlikely to happen as that use case has already gone to lengths to opt out of reactivity in the first place, so I expect this to be nothing but a performance improvement for those use cases if they exist.

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:
I am unable to build or run tests. The development setup instructions are insufficient to get a dev environment working due to #4338.