message: don't overwrite callback with undefined if no customization provided
diff: @@ -296,10 +296,11 @@ class _MatrixClientPeg implements IMatrixClientPeg {
  // These are always installed regardless of the labs flag so that
  // cross-signing features can toggle on without reloading and also be
  // accessed immediately after login.
  - const customisedCallbacks = {
  - getDehydrationKey: SecurityCustomisations.getDehydrationKey,
  - };
  - Object.assign(opts.cryptoCallbacks, crossSigningCallbacks, customisedCallbacks);
  + Object.assign(opts.cryptoCallbacks, crossSigningCallbacks);
  + if (SecurityCustomisations.getDehydrationKey) {
  + opts.cryptoCallbacks.getDehydrationKey =
  + SecurityCustomisations.getDehydrationKey;
  + }
  this.matrixClient = createMatrixClient(opts);
  