Warn on empty `aria-` attributes

[Rich-Harris]

Describe the problem

Svelte warns on this...

...but not on this, which is no more valid:

We're not 'fixing' things behind the scenes either — that'll become aria-hidden="", which is straightforwardly incorrect.

Describe the proposed solution

Stop treating empty aria-x attributes as equivalent to aria-x="true". They're not.

Importance

nice to have