[in_app_purchase] Prepare for NULLABLE_ARGUMENT_TO_NON_NULL_TYPE

[asashour]

flutter/flutter#121318

This PR modifies the code in order not to trigger the incoming diagnostic.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[asashour]

The false positive linter error is fixed in Dart 3.0.0-264.0.dev, so I think this PR needs to wait a little.

[Hixie]

test-exempt: fixing lint

This is going to be tricky to land though because it has to be lint-free on both the main branch and the last stable. Search for _ambiguate for ways we've handled that before.

[stuartmorgan-g]

LGTM other than the comment wording

[stuartmorgan-g]

@bparrishMines or @tarrinneal could you do the second review here?