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/rows.js

@@ -74,7 +74,7 @@ exports.is_draft_row = function (row) {
 exports.id = function (message_row) {
     if (exports.is_draft_row(message_row)) {
         blueslip.error("Drafts have no zid");
-        return;
+        return undefined;
     }
 
     /*
@@ -103,7 +103,7 @@ exports.local_echo_id = function (message_row) {
 
     if (zid === undefined) {
         blueslip.error("Calling code passed rows.local_id a row with no zid attr.");
-        return;
+        return undefined;
     }
 
     if (!zid.includes(".0")) {