Uri.origin should never throw about sdk

[github-actions]

area-core-library, type-enhancement

The Uri.origin method throws an exception when the scheme or host properties are empty or null. This behavior is unexpected and can lead to bugs, especially when dealing with unknown URIs. The user proposes that Uri.origin should always return a value, even if the scheme or host is missing, to avoid unexpected exceptions.

[lrhn]

It's not technically a breaking change to make something not throw an error, but if the current origin behavior is being used as a way to check if two different URIs have the same origin, then returning an empty string instead of an error may cause false positives.

The Dart Uri.origin matches neither the HTML behavior nor the WhatWG URL behavior, so there is no real specification that it's failing to follow. It's not even trying.

I think returning the string "null" would be a reasonable (and semi-compliant) behavior. Maybe we should do that.