[Backport staging] Fix default behavior of Room.getBlacklistUnverifiedDevices

spec/unit/room.spec.ts

@@ -2737,4 +2737,15 @@ describe("Room", function() {
             expect(room.getPendingEvent(ev.getId())).toBe(ev);
         }
     });
+
+    describe("getBlacklistUnverifiedDevices", () => {
+        it("defaults to null", () => {
+            expect(room.getBlacklistUnverifiedDevices()).toBeNull();
+        });
+
+        it("is updated by setBlacklistUnverifiedDevices", () => {
+            room.setBlacklistUnverifiedDevices(false);
+            expect(room.getBlacklistUnverifiedDevices()).toBe(false);
+        });
+    });
 });

src/crypto/algorithms/megolm.ts

@@ -1134,8 +1134,9 @@ class MegolmEncryption extends EncryptionAlgorithm {
 
         // The global value is treated as a default for when rooms don't specify a value.
         let isBlacklisting = this.crypto.getGlobalBlacklistUnverifiedDevices();
-        if (typeof room.getBlacklistUnverifiedDevices() === 'boolean') {
-            isBlacklisting = room.getBlacklistUnverifiedDevices();
+        const isRoomBlacklisting = room.getBlacklistUnverifiedDevices();
+        if (typeof isRoomBlacklisting === 'boolean') {
+            isBlacklisting = isRoomBlacklisting;
         }
 
         // We are happy to use a cached version here: we assume that if we already

src/models/room.ts

@@ -1325,8 +1325,9 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
      * @return {Boolean} true if blacklisting unverified devices, null
      * if the global value should be used for this room.
      */
-    public getBlacklistUnverifiedDevices(): boolean {
-        return !!this.blacklistUnverifiedDevices;
+    public getBlacklistUnverifiedDevices(): boolean | null {
+        if (this.blacklistUnverifiedDevices === undefined) return null;
+        return this.blacklistUnverifiedDevices;
     }
 
     /**