Enable nullable reference types in WaitForAppDetection.cs

[Copilot]

Opts the shipped WaitForAppDetection MSBuild task into nullable reference types, making explicit the existing contract that its single field can legitimately be null. Follows the conversion of its sibling DetectIfAppWasUninstalled.cs (#11598), which deferred this file.

Changes

• Added #nullable enable as the first line of src/Xamarin.Android.Build.Debugging.Tasks/Tasks/WaitForAppDetection.cs.
• Marked getPackagesAsync nullable, reflecting that GetRegisteredTaskObjectAssemblyLocal<T> returns null when nothing is registered:

System.Threading.Tasks.Task<List<AndroidInstalledPackage>>? getPackagesAsync;

The existing if (getPackagesAsync == null) return; guard in RunTaskAsync () already covers the null case, so no further changes were needed. No null-forgiving (!) operator is used; the project targets netstandard2.0, so ArgumentNullException.ThrowIfNull is not applicable.

[Copilot]

Pull request overview

Opts the shipped WaitForAppDetection MSBuild task into nullable reference types to accurately represent that its cached getPackagesAsync task may be absent when no task object is registered.

Changes:

• Enabled nullable reference types in WaitForAppDetection.cs via #nullable enable.
• Marked the getPackagesAsync field as nullable to match the existing null guard in RunTaskAsync ().