[path_provider_linux] Migrate to nnbd

[gaetschwartz]

Migrating path_provider_linux to null-safety. Needed to migrate shared_preferences.

It contributes to fixing flutter/flutter#74876.

Pre-launch Checklist

• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
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• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

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[gaetschwartz]

I'm unsure as to why the CI is failing, it's related to path_provider_windows, which I didn't touch.

[stuartmorgan]

There's an existing PR for this at #3330; please comment there if you have changes to suggest.

[gaetschwartz]

@stuartmorgan What is wrong with the other PR ? It's currently blocking the migration of shared_preferences, one of the most used package of this repo. Getting it reviewed would help a lot :)

[stuartmorgan]

@stuartmorgan What is wrong with the other PR ?

I'm not sure what you are asking here. Why do you think something is wrong with it?

It's currently blocking the migration of shared_preferences, one of the most used package of this repo. Getting it reviewed would help a lot :)

I expect to review it next week. It's only actually been reviewable for about a day, since a core dependency hadn't been migrated.

[gaetschwartz]

@stuartmorgan Alright, was just wondering why it hadn't been reviewed. Missed the part where it needed a code dependency 😊 Sorry if it sounded weird, wasn't the point 💙☺️