eslint: Replace empty-returns with consistent-return.

Instead of prohibiting ‘return undefined’ (#8669), we require that a
function must return an explicit value always or never.  This prevents
you from forgetting to return a value in some cases.  It will also be
important for TypeScript, which distinguishes between undefined and
void.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/js/compose.js

@@ -403,7 +403,7 @@ exports.finish = function () {
     if (zcommand.process(message_content)) {
         exports.do_post_send_tasks();
         clear_compose_box();
-        return;
+        return undefined;
     }
 
     if (!exports.validate()) {