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/poll_widget.js

@@ -118,7 +118,7 @@ class PollData {
                 if (this.is_my_poll) {
                     return event;
                 }
-                return;
+                return undefined;
             },
 
             inbound: (sender_id, data) => {