qga/qapi-schema: Fix guest-set-memory-blocks documentation

Documentation claims the command can "return NULL".  "NULL" doesn't
exist in JSON.  "null" does, but the command returns lists, and null
isn't.  Correct documentation to "return an empty list".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240227113921.236097-13-armbru@redhat.com>

qga/qapi-schema.json

@@ -1176,9 +1176,9 @@
 #     @GuestMemoryBlockResponse, which is corresponding to the input
 #     list.
 #
-#     Note: it will return NULL if the @mem-blks list was empty on
-#     input, or there is an error, and in this case, guest state will
-#     not be changed.
+#     Note: it will return an empty list if the @mem-blks list was
+#     empty on input, or there is an error, and in this case, guest
+#     state will not be changed.
 #
 # Since: 2.3
 ##