fix(#11040): Add a warning when binding to object with empty keys

[CallumChaney]

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:
With this PR binding to an object with an empty key value will still cause Vue to crash just adds a clearer warning message as to why. The warning message could probably be even clearer.

[posva]

Thanks but this should be done only when using v-bind="{'': 'foo'}, not directly in the observer code. A test is also needed

[CallumChaney]

@posva ahh ok, sorry I'm still pretty new to Vue code base, could you point me towards the best place to make the change then I'm thinking the bind-object-props render-helper? I was also thinking that exiting baseSetAtrritube if the empty key is detected would prevent the crash, just want to make sure I'm tackling this bug right. Thanks