Don't log exceptions for obviously incorrect stream tokens (#18139)

We log incorrect ones as we want to catch bugs where Synapse returns bad
tokens. However, sometimes clients just send tokens that are e.g. empty.

---------

Co-authored-by: Eric Eastwood <erice@element.io>

changelog.d/18139.misc

+Do not log at the exception-level when clients provide empty `since` token to `/sync` API.

synapse/types/__init__.py

@@ -664,6 +664,11 @@ class RoomStreamToken(AbstractMultiWriterStreamToken):
 
     @classmethod
     async def parse(cls, store: "PurgeEventsStore", string: str) -> "RoomStreamToken":
+        # Check that it looks like a Synapse token first. We do this so that
+        # we don't log at the exception-level for obviously incorrect tokens.
+        if not string or string[0] not in ("s", "t", "m"):
+            raise SynapseError(400, f"Invalid room stream token {string:!r}")
+
         try:
             if string[0] == "s":
                 return cls(topological=None, stream=int(string[1:]))