drafts: Catch and log error caused by drafts with undefined topics.

This is a bandaid fix to an error that came out of #22094.
The error was fixed in #23238 but some drafts still have
undefined topics which is raising this error.

More details on CZO: https://chat.zulip.org/#narrow/stream/9-issues/topic/live.20update.20issue.20with.20drafts/near/1457913

static/js/util.js

@@ -1,5 +1,6 @@
 import _ from "lodash";
 
+import * as blueslip from "./blueslip";
 import {$t} from "./i18n";
 
 // From MDN: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Math/random
@@ -39,6 +40,10 @@ export function lower_bound(array, value, less) {
 }
 
 export const lower_same = function lower_same(a, b) {
+    if (a === undefined || b === undefined) {
+        blueslip.error(`Cannot compare strings; at least one value is undefined: ${a}, ${b}`);
+        return false;
+    }
     return a.toLowerCase() === b.toLowerCase();
 };