Enable nullable reference types in DetectIfAppWasUninstalled.cs

[Copilot]

Enable #nullable enable in DetectIfAppWasUninstalled.cs and apply standard nullable conventions.

• Non-required properties (AdbTarget, UserID) → string?
• [Required] properties (PackageName, UploadFlagFile) → string with = "" default
• QueryPackages return type → Task<List<AndroidInstalledPackage>?> (returns null in catch)

Project targets netstandard2.0, so no ArgumentNullException.ThrowIfNull usage.

[Copilot]

Pull request overview

This PR enables nullable reference types in DetectIfAppWasUninstalled.cs (a debugging MSBuild task) and updates task properties/method signatures to follow the repo’s nullable conventions for netstandard2.0.

Changes:

• Added #nullable enable to opt the file into nullable analysis.
• Updated task properties: non-required inputs (AdbTarget, UserID) to string?, and [Required] inputs (PackageName, UploadFlagFile) to non-nullable string with = "" defaults.
• Adjusted QueryPackages signature to return a nullable list (currently Task<List<AndroidInstalledPackage>?>).

Comments suppressed due to low confidence (1)

src/Xamarin.Android.Build.Debugging.Tasks/Tasks/DetectIfAppWasUninstalled.cs:65

• ❌ Changing QueryPackages to return Task<List<AndroidInstalledPackage>?> breaks the registered task object type: WaitForAppDetection retrieves it as Task<List<AndroidInstalledPackage>> (see WaitForAppDetection.cs:16-21), so the lookup can fail due to the generic type mismatch and the wait will be skipped. Also, in nullable context User = UserID will produce a nullability warning because PmListPackagesCommand.User is a non-nullable string. Keep the return type non-nullable, coalesce UserID to "", and return an empty list on failure instead of null.

		async System.Threading.Tasks.Task<List<AndroidInstalledPackage>?> QueryPackages (AndroidDevice device, string uploadFlagFileFullPath)
		{
			// DO NOT use the Log.XXXX methods in this method.
			// Because this is running on a background thread they will
			// end up locking the UI in VS.