Fix a potential bug of UnboundLocalError (#8329)

Replaced with less buggier control flow

changelog.d/8329.bugfix

@@ -0,0 +1 @@
+Fix UnboundLocalError from occuring when appservices send malformed register request.

synapse/rest/client/v2_alpha/register.py

@@ -431,11 +431,14 @@ async def on_POST(self, request):
 
             access_token = self.auth.get_access_token_from_request(request)
 
-            if isinstance(desired_username, str):
-                result = await self._do_appservice_registration(
-                    desired_username, access_token, body
-                )
-            return 200, result  # we throw for non 200 responses
+            if not isinstance(desired_username, str):
+                raise SynapseError(400, "Desired Username is missing or not a string")
+
+            result = await self._do_appservice_registration(
+                desired_username, access_token, body
+            )
+
+            return 200, result
 
         # == Normal User Registration == (everyone else)
         if not self._registration_enabled: