Return null from decryptEvent if session is unknown

This just makes the shape of the API a bit saner.

lib/crypto/index.js

@@ -869,8 +869,9 @@ Crypto.prototype.decryptEvent = function(event) {
         olmDevice: this._olmDevice,
     });
     var r = alg.decryptEvent(event);
-    var payload = r.result;
-    if (r.sessionExists) {
+
+    if (r !== null) {
+        var payload = r.result;
         payload.keysClaimed = r.keysClaimed;
         payload.keysProved = r.keysProved;
         return payload;