Inconsistent marking of intermediary site as maybe-`undefined`

[apasel422]

It is initialized in

attribution/api.bs

Lines 1446 to 1451 in 85e2869

	1. Let |intermediarySite| be:
	1. a value of `undefined` if the [=origin=] is [=same site=]
	with the [=top-level origin=],
	1. otherwise, the result of
	[=obtain a site|obtaining a site=]
	from |settings|' [=environment settings object/origin=].

and passed to the do attribution algorithm, which doesn't indicate that it accepts undefined:

attribution/api.bs

Lines 1598 to 1602 in 85e2869

	To <dfn>do attribution and fill a histogram</dfn>, given
	[=validated conversion options=] |options|,
	[=site=] |topLevelSite|,
	[=site=] |intermediarySite|,
	and [=moment=] |now|:

Same for the common matching logic:

attribution/api.bs

Lines 1688 to 1692 in 85e2869

	To perform <dfn>common matching logic</dfn>, given
	[=validated conversion options=] |options|,
	[=site=] |topLevelSite|,
	[=site=] |intermediarySite|,
	[=epoch index=] |epoch|, and [=moment=] |now|: