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.

Author: jgraham

index.bs

@@ -394,8 +394,11 @@ Each [=command=] is defined by:
 - A <dfn export for=command>result type</dfn>, which is defined by a [=local
   end definition=] fragment.
 - A set of [=remote end steps=] which define the actions to take for a command
-  given a [=BiDi session=] and [=command parameters=] and return an
-  instance of the command [=result type=].
+  given a [=BiDi session=] and [=command parameters=]. If the command runs to
+  completion these steps return [=success=] with either a [=map=] matching the
+  command [=result type=] production or with null to indicate an
+  `EmptyResult`. If the command does not run to completion these steps return
+  [=error=] with the [=error code=] that will be reported to the user.
 
 A command that can run without an active session is a <dfn>static
 command</dfn>. Commands are not static commands unless stated in their
@@ -797,6 +800,8 @@ 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|.
 
@@ -4917,7 +4922,7 @@ The [=remote end steps=] given |session| and |command parameters| are:
 
 1. <a for=set>Remove</a> |script| from |preload script map|.
 
-1. Return null
+1. Return [=success=] with data null
 
 </div>