Fix returning null from commands

We want returning null to map onto an empty result. We could
do this explicitly per command, but it's less verbose to just
allow null at the top level.

index.bs

@@ -797,6 +797,8 @@ To <dfn>handle an incoming message</dfn> given a [=WebSocket connection=]
 
     1. Let |value| be |result|'s data.
 
+    1. If |value| is null, set |value| to an empty map.
+
     1. Assert: |value| matches the definition for the [=result type=]
        corresponding to the command with [=command name=] |method|.