Fix key verification when key stored with null valid_until_ms

Some keys are stored in the synapse database with a null valid_until_ms
which caused an exception to be thrown when using that key. We fix this
by treating nulls as zeroes, i.e. they keys will match verification
requests with a minimum_valid_until_ms of zero (i.e. don't validate ts)
but will not match requests with a non-zero minimum_valid_until_ms.

Fixes #5391.

synapse/storage/keys.py

@@ -80,6 +80,14 @@ class KeyStore(SQLBaseStore):
 
             for row in txn:
                 server_name, key_id, key_bytes, ts_valid_until_ms = row
+
+                if ts_valid_until_ms is None:
+                    # Old keys may be stored with a ts_valid_until_ms of null,
+                    # in which case we treat this as if it was set to `0`, i.e.
+                    # it won't match key requests that define a minimum
+                    # `ts_valid_until_ms`.
+                    ts_valid_until_ms = 0
+
                 res = FetchKeyResult(
                     verify_key=decode_verify_key_bytes(key_id, bytes(key_bytes)),
                     valid_until_ts=ts_valid_until_ms,