Don't run migration for Rust crypto if the legacy store is empty (#4218)

* Don't run migration for Rust crypto if the legacy store is empty

Fixes https://github.com/element-hq/element-web/issues/27447

* Add copyright for the TypeScript files in legacy DB dumps

* Provide a type for the accountPickle we check for before migration

* Remove redundant backup response

This is unused

* Simplify keys response

* Downgrade log message.

---------

Co-authored-by: Richard van der Hoff <richard@matrix.org>

spec/test-utils/test_indexeddb_cryptostore_dump/unverified/index.ts

@@ -1,3 +1,19 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
 import { DumpDataSetInfo } from "../index";
 
 /**