The URL spec represents its path as a:
Variant<String, Vector<String>>
A URL is defined has having an opaque path if it has a single String,
the URL path otherwise containing a list of path components.
We (like in an older version of the spec) track this through a boolean
and only use a Vector with a single component for opaque paths.
This means it was incorrect to simple assign the path to a list with
a single empty string without setting that URL as opaque, which
meant that the path serialization was producing incorrect results.
It may make sense changing the API so this situation is a little more
clear. But for now, we simply need to set the opaque path boolean
to true here.
