[xdg_directories] Migrate xdg_directories to nnbd

[yash1200]

This PR migrates xdg_directories package to null safety.

Fixes

• Audit code in flutter/packages/* for null safety flutter#71506
• Migrate the xdg_directories package to null safety. flutter#74836

Related PR

• [xdg_directories] Migrate to null safety #250

Pre-launch Checklist

• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[yash1200]

@stuartmorgan How can i write test for this check you referenced here #250 (comment)

[stuartmorgan-g]

@stuartmorgan How can i write test for this check you referenced here #250 (comment)

https://api.flutter.dev/flutter/package-matcher_matcher/isNotNull-constant.html

[yash1200]

@stuartmorgan
Do you mean something like this?

expect(xdg.runtimeDir, isNotNull);
expect(xdg.runtimeDir!.path, equals(testPath('.local/test_runtime')));

[stuartmorgan-g]

Yes, that should be fine. (I was thinking that you'd need to wrap the second line in a null check, but it looks like unlike C++ gtest expectations, failures in Dart's expect immediately kill the test.)

[yash1200]

Done the requried changes
@stuartmorgan

[stuartmorgan-g]

LGTM, thanks!