<a href={null}> sets href="null" rather than omitting the attribute

[chris-morgan]

Trivial reproduction case:

<a href={null} something-else={null}>Alas!</a>

In the compiled JavaScript:

			a.href = null;
			setAttribute(a, "something-else", null);

Although a.href = null; is shorter than setAttribute(a, "href", null), it’s incorrect, because setting HTMLAnchorElement.prototype.href stringifies its argument, including null, and so you end up with <a href="null>.

It’s not the most common situation to craft an <a> which may or may not have an href, but I just hit this now.

[Conduitry]

This is a duplicate of the (admittedly slightly meandering) #1434. This has been an issue since the very beginning of Svelte. Rich doesn't remember why he decided to use properties instead of attributes where possible (i.e., whether it was a performance thing or a code size thing). My comment on that issue from January 10 has a workaround. But this is still a known bug, just one not on the front burning right now amid v3 stuff.