fix(last-n-limits): crash on undefined

react/features/base/lastn/functions.js

  * of participants or {@code undefined} otherwise.
  */
 export function limitLastN(participantsCount, lastNLimits) {
+    if (!lastNLimits || !lastNLimits.keys) {
+        return undefined;
+    }
+
     let selectedLimit;
 
     for (const participantsN of lastNLimits.keys()) {

react/features/base/lastn/functions.test.js

 import { limitLastN, validateLastNLimits } from './functions';
 
-describe('limitsLastN', () => {
+describe('limitLastN', () => {
+    it('handles undefined mapping', () => {
+        expect(limitLastN(0, undefined)).toBe(undefined);
+    });
     describe('when a correct limit mapping is given', () => {
         const limits = new Map();