Thank you!

After some thinking. While we're at this, we should probably enrich that method in general. According (if I read it correct) to RFC 9110:

Host = uri-host [ ":" port ]; Section 4

uri-host      = <host, see [URI], Section 3.2.2>
port          = <port, see [URI], Section 3.2.3>

Host part is mandatory. And, either way, I think it's at least mandatory in case of "default" ))

So, I think:

• ":123" (empty host, different port, as in https://:123/foobar) MUST be invalid for default host header
• "" (empty host, no whitespaces, same port, as in https:///foobar) MUST be invalid as well

cc: @sferik