[image_picker] Migrate Image_picker_platform interface to Null Safety

[Sameerkash]

Migrate Image_picker_platform interface to Null Safety

Related to #75162

All existing tests are passing

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.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• 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.

[Hixie]

test-exempt: refactoring with no logical change

[Sameerkash]

@stuartmorgan Any hints on the failing checks? is it the CI config?

[cyanglaz]

@Sameerkash Thank you for the PR! This is being done in #3492 and we are almost landing the PR. I'm going to close this one :)

[cyanglaz]

@Sameerkash If you'd like to work on the migration for the image_picker plugin after #3492, I'll be super happy to review it! :)

[stuartmorgan]

@Sameerkash Apologies for the duplicated work here; I tried to capture in-progress work when filing all of the bugs, but missed #3492 when filing that issue.

[Sameerkash]

@Sameerkash If you'd like to work on the migration for the image_picker plugin after #3492, I'll be super happy to review it! :)

Yes. I would like to @cyanglaz
I'll start asap. Should be up in a day.

[Sameerkash]

@Sameerkash Apologies for the duplicated work here; I tried to capture in-progress work when filing all of the bugs, but missed #3492 when filing that issue.

It's okay!

[cyanglaz]

Yes. I would like to @cyanglaz
I'll start asap. Should be up in a day.

Please tag me on the PR! I'll be happy to review it!