Add Unit tests for `CrowdinStorage` class

Currently, this class is not covered by tests - [Codecov report](https://app.codecov.io/gh/crowdin/flutter-sdk/blob/main/lib/src/crowdin_storage.dart)

Class - [crowdin_storage.dart](https://github.com/crowdin/flutter-sdk/blob/main/lib/src/crowdin_storage.dart)

Tests examples - https://github.com/crowdin/flutter-sdk/tree/main/test

Start with lib/src/crowdin_storage.dart and review the existing tests in test for patterns used by the SDK. Use the linked Codecov report to identify uncovered behavior, then add unit tests for CrowdinStorage and run the relevant test suite. Done means the class has meaningful test coverage.

Written by the indexing model from the issue text.