Boolean props conversions don't work

[jods4]

Version

3.0.0-alpha.8

Reproduction link

https://codesandbox.io/s/kind-franklin-cq3le

Steps to reproduce

Define a boolean prop and use it as an empty attribute

What is expected?

props should be true

What is actually happening?

props is the empty string

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I have a PR ready

[jods4]

@yyx990803 I just noticed another issue with the boolean conversion that my PR didn't fix.
Code works properly but both tests and runtime produce the following warning:

Invalid prop: type check failed for prop "active". Expected Boolean, got String with value "".

[jods4]

Also I learned a new trick today. It's more obscure but maybe more efficent, >>> always returns a positive integer, even in edge case >>> 0. So -1 >>> 0 == 4294967295

So where my PR did
stringIndex < 0 || booleanIndex < stringIndex
you could do, if you're into bit tricks:
booleanIndex < (stringIndex >>> 0)
It might be shorter code and maybe more efficient, it's definitely more obscure.
Brackets are not required and would be removed by minification, but they make the expression easier to understand.

[yyx990803]

Fixed the boolean validation issue in 3b282e7.
Re the trick: I think we should stick to something that's easier to understand.