RawLabel: Fix crash when text is empty string.

[WesleyAC]

This was causing a crash when trying to autocomplete a user group with a
blank name. The empty string was passed into RawLabel, which caused this
invariant to incorrectly trigger.

Instead of using !! to check for undefined, we can do != null, which
will trigger for undefined and null, but not an empty string.

If you want to trigger this on CZO, just type @test into the composebox :)

[chrisbobbe]

Thanks for catching this!! One tiny correctness hole that I think I've found, below; possibly not worth addressing, but also wouldn't be too hard to address if you chose to.

[chrisbobbe]

It occurs to me that children can be a number or a string; it's typed ?Node (link) and I think the type Flow uses for Node includes strings and numbers (doc).

It'd be super weird, but I guess one could pass "foo" for text and 0 for children, and circumvent this check, ending up with "foo0" on the screen?

I'm trying to think whether that case is totally wacky and you'd really have to be trying to find it in order for it to be active. Or if you could stumble on it innocently.

I think it's not totally impossible to stumble on it innocently: there's this gotcha in this conditional rendering doc from React:

Note that returning a falsy expression will still cause the element after && to be skipped but will return the falsy expression. In the example below, <div>0</div> will be returned by the render method.

If you're passing children and you've accidentally stumbled on that gotcha there, and you're also passing a string for text, there could be a case like foo0.

[gnprice]

Yeah, seems cleanest to use the same != null for children as well.

Thanks @WesleyAC for catching this!

[WesleyAC]

Revision pushed :)

[gnprice]

Looks good, thanks -- merged!