Fix destination_is errors seen in sentry.

[DMRobertson]

Fixes #12690.

Two problems:

1. If destination is the empty bytstring and destination_is is None, the following line raises a KeyError.
   

   synapse/synapse/http/matrixfederationclient.py

   Line 748 in 8dd3e0e

   request.get("destination") or request["destination_is"],

2. The destination should never be an empty bytestring---we should catch and reject it earlier. Sentry reports seeing this from the ProfileRestServlet, which does try to validate a user ID:
   

   synapse/synapse/rest/client/profile.py

   Lines 144 to 153 in 94b620a

   	async def on_GET(
   	self, request: SynapseRequest, user_id: str
   	) -> Tuple[int, JsonDict]:
   	requester_user = None
   	if self.hs.config.server.require_auth_for_profile_requests:
   	requester = await self.auth.get_user_by_req(request)
   	requester_user = requester.user
   	user = UserID.from_string(user_id)

   
   But the validation logic isn't watertight. Fixing it up is easy, but I'm a bit scared that it might cause unintended side-effects elsewhere.

Prior to #11398, Synapse would happily create an Authorization: X-Matrix header for a request blob whose destination and/or destination_isfields were the empty string. My understanding is that such an authorization header is useless: the recipient of this header does not know who owns the message claims to be from, so we cannot confirm they control the private key corresponding to the given public key in the header.

[clokep]

Fairly certain this looks fine.

[DMRobertson]

SonarCloud complains that there is duplication. I didn't like the repeated calls to is_valid either, SonarCloud, but I've learned to live with my sins.

[clokep]

Seems good if tests pass.