Suppress empty body warnings in room servelets (#14600)

* Suppress empty body warnings in room servelets

We've already decided to allow empty bodies for backwards compat. The
change here stops us from emitting a misleading warning; see also
#14478 (comment)

* Changelog

changelog.d/14600.bugfix

@@ -0,0 +1 @@
+Suppress a spurious warning when `POST /rooms/<room_id>/<membership>/`, `POST /join/<room_id_or_alias`, or the unspecced `PUT /join/<room_id_or_alias>/<txn_id>` receive an empty HTTP request body.

synapse/rest/client/room.py

@@ -396,12 +396,7 @@ async def on_POST(
     ) -> Tuple[int, JsonDict]:
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
-        try:
-            content = parse_json_object_from_request(request)
-        except Exception:
-            # Turns out we used to ignore the body entirely, and some clients
-            # cheekily send invalid bodies.
-            content = {}
+        content = parse_json_object_from_request(request, allow_empty_body=True)
 
         # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
         args: Dict[bytes, List[bytes]] = request.args  # type: ignore
@@ -952,12 +947,7 @@ async def on_POST(
         }:
             raise AuthError(403, "Guest access not allowed")
 
-        try:
-            content = parse_json_object_from_request(request)
-        except Exception:
-            # Turns out we used to ignore the body entirely, and some clients
-            # cheekily send invalid bodies.
-            content = {}
+        content = parse_json_object_from_request(request, allow_empty_body=True)
 
         if membership_action == "invite" and all(
             key in content for key in ("medium", "address")