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/resize.js

@@ -119,7 +119,7 @@ exports.watch_manual_resize = function (element) {
 
         if (!box) {
             blueslip.error("Bad selector in watch_manual_resize: " + element);
-            return;
+            return undefined;
         }
 
         const meta = {