Fix for FilePicker PickMultipleAsync nullable reference type

[SuthiYuvaraj]

RootCause

FilePicker.PickMultipleAsync() has incorrect nullable reference type annotations causing compiler warnings with #nullable enable. When checking if the result is null, the compiler warns "Expression is always true" because IEnumerable<FileResult?> is not marked nullable, even though the method returns null.

Reason for Regression:

PR #27961 attempted to fix null return behavior but applied the annotation incorrectly, making items nullable (Task<IEnumerable<FileResult?>>) instead of making the collection nullable (Task<IEnumerable<FileResult>?>).

Description of Change:

The fix moves the nullable marker from collection items to the collection itself, changing Task<IEnumerable<FileResult?>> to Task<IEnumerable<FileResult>?> in the IFilePicker interface, FilePicker static class, and FilePickerImplementation class.

Issues Fixed

Fixes #33114

Tested the behaviour on the following platforms

• Android
• Windows
• iOS
• Mac

Note:

Test Case: Not applicable. This fix corrects nullable annotations only and does not change runtime behavior. The scenario cannot be validated through automated tests.

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Pull request overview

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