[SSR] aria-* attributes of dynamic null/undefined values render as strings

[nolanlawson]

<div aria-label={dynamic}></div>
<x-child aria-label={dynamic}></x-child>

As demonstrated in #4787, if dynamic is undefined, then this renders (in engine-server):

<div></div>
<x-child aria-activedescendant="undefined"></x-child>

And for null:

<div></div>
<x-child aria-activedescendant="null"></x-child>

This is inconsistent at best and wrong at worst. We should probably omit the attribute in both cases.

[wjhsf]

Is this specific to ARIA attributes? All of them, standard, or non-standard? Does it impact regular attributes, global HTML attributes, @api attributes?

[nolanlawson]

Is this specific to ARIA attributes?

Dunno. Worth testing HTML global attributes and data-foo as well.

All of them, standard, or non-standard?

In this case the attributes are purely being used as attributes, so "standard vs nonstandard" doesn't apply. E.g. we have aria-labelledby here as an attribute, not our nonstandard ariaLabelledBy property.

@api

This would be another good thing to add tests for. I don't believe it should matter, but you never know.

[nolanlawson]

BTW this is not terribly important because, unless you have an element with id="undefined" or id="null" somewhere, then the effective difference between rendering aria-labelledby="undefined" and not rendering it at all is zero.