ignore empty domain for digets auth

[wfurt]

rfc7616 suggest that missing or empty should be treated equally.

  domain

      A quoted, space-separated list of URIs, as specified in [RFC3986],
      that define the protection space.  If a URI is a path-absolute, it
      is relative to the canonical root URL.  (See Section 2.2 of
      [RFC7235].)  An absolute-URI in this list may refer to a different
      server than the web-origin [RFC6454].  The client can use this
      list to determine the set of URIs for which the same
      authentication information may be sent: any URI that has a URI in
      this list as a prefix (after both have been made absolute) MAY be
      assumed to be in the same protection space.  If this parameter is
      omitted or its value is empty, the client SHOULD assume that the
      protection space consists of all URIs on the web-origin.

fixes #50283

[ghost]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

rfc7616 suggest that missing or empty should be treated equally.

  domain

      A quoted, space-separated list of URIs, as specified in [RFC3986],
      that define the protection space.  If a URI is a path-absolute, it
      is relative to the canonical root URL.  (See Section 2.2 of
      [RFC7235].)  An absolute-URI in this list may refer to a different
      server than the web-origin [RFC6454].  The client can use this
      list to determine the set of URIs for which the same
      authentication information may be sent: any URI that has a URI in
      this list as a prefix (after both have been made absolute) MAY be
      assumed to be in the same protection space.  If this parameter is
      omitted or its value is empty, the client SHOULD assume that the
      protection space consists of all URIs on the web-origin.

fixes #50283

Author:	wfurt
Assignees:	wfurt
Labels:	area-System.Net.Http
Milestone:	-

[wfurt]

/backport to release/5.0

[github-actions]

Started backporting to release/5.0: https://github.com/dotnet/runtime/actions/runs/709482438