Fix overriding undefined per-room unverified devices setting

If the per-room setting for preventing sending keys to unverified
devices is `undefined`, it overrides the global setting (which
could be `true`).

src/crypto/algorithms/megolm.js

@@ -543,7 +543,7 @@ MegolmEncryption.prototype._getDevicesInRoom = function(room) {
 
     // The global value is treated as a default for when rooms don't specify a value.
     let isBlacklisting = this._crypto.getGlobalBlacklistUnverifiedDevices();
-    if (room.getBlacklistUnverifiedDevices() !== null) {
+    if (typeof room.getBlacklistUnverifiedDevices() === 'boolean') {
         isBlacklisting = room.getBlacklistUnverifiedDevices();
     }