Export crawling with missing/empty hrefs

[Conduitry]

sapper export will crash in certain situations involving missing or empty hrefs on links.

function get_href(attrs: string) {
	const match = /href\s*=\s*(?:"(.*?)"|'(.+?)'|([^\s>]+))/.exec(attrs);
	return match[1] || match[2] || match[3];
}

If an <a> is missing an href altogether, this will crash. If it has href="" it's fine and will return an empty string, but if it has href='' it will return an incorrect string including the quotes (because '(.+?)' refuses to match the pattern, and so it falls back to the third option). I think<a href=> or <a href= > would also fail, but I'm not actually sure what the HTML spec says about that.

[Conduitry]

Currently proposed fix: Change the +s to *s in the regex so that it will match empty hrefs, and also check that match is not null (and return null in that case) before trying to subscript it.