Improve nullability.

[rolfbjarne]

Sprinkle nullability information in a number of places where it makes sense.

[Copilot]

Pull request overview

This PR adds/adjusts C# nullable annotations across XHarness shared iOS and common components to better model where values may legitimately be absent (e.g., platform/configuration and environment variables), improving static null-safety across the codebase.

Changes:

• Made many platform/configuration parameters nullable in ProjectFileExtensions APIs.
• Updated process execution APIs to allow environment variable values to be nullable (Dictionary<string, string?>), enabling explicit removal of variables.
• Updated logging and simulator-selection APIs with additional nullability annotations.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Microsoft.DotNet.XHarness.iOS.Shared/Utilities/ProjectFileExtensions.cs	Marks platform/configuration inputs as nullable across project XML helpers.
src/Microsoft.DotNet.XHarness.iOS.Shared/IResultParser.cs	Updates result parser API to allow nullable variation.
src/Microsoft.DotNet.XHarness.iOS.Shared/Hardware/SimulatorLoader.cs	Adjusts simulator selection enumerable nullability.
src/Microsoft.DotNet.XHarness.iOS.Shared/Execution/MlaunchProcessManager.cs	Allows nullable env-var values in mlaunch process execution APIs.
src/Microsoft.DotNet.XHarness.iOS.Shared/Execution/IMlaunchProcessManager.cs	Keeps interface in sync with mlaunch manager env-var nullability changes.
src/Microsoft.DotNet.XHarness.Common/Logging/NullLog.cs	Accepts nullable strings/args in the no-op logger implementation.
src/Microsoft.DotNet.XHarness.Common/Logging/Log.cs	Broadens logger write APIs and WriteImpl nullability.
src/Microsoft.DotNet.XHarness.Common/Logging/ILog.cs	Broadens logger interface to accept nullable strings/args.
src/Microsoft.DotNet.XHarness.Common/Execution/ProcessManager.cs	Allows nullable env-var values (with removal behavior when null).
src/Microsoft.DotNet.XHarness.Common/Execution/IProcessManager.cs	Keeps interface in sync with ProcessManager env-var nullability changes.
src/Microsoft.DotNet.XHarness.Apple/AppOperations/AppRunnerBase.cs	Allows passing nullable env-var dictionary into Mac Catalyst app runner.

Comments suppressed due to low confidence (1)

src/Microsoft.DotNet.XHarness.Common/Logging/Log.cs:27

• Write(string? value) can pass null through to WriteImpl (e.g., when Timestamp is false and value is null). Many existing WriteImpl overrides currently take a non-null string and may warn (nullability mismatch) or throw if they start receiving null. Consider coalescing value to string.Empty before calling WriteImpl (and keep WriteImpl non-nullable), or update all overrides to accept/handle string? safely.

    public void Write(string? value)
    {
        if (Timestamp)
        {
            value = "[" + DateTime.Now.ToString("HH:mm:ss.fffffff") + "] " + value;

[Copilot]

GetElementValue now allows platform/configuration to be null, but the method body later uses these values in string replacements (e.g., Replace("$(Platform)", platform) / Replace("$(Configuration)", configuration)) which will throw if the replacement value is null. Update the implementation to only replace when the corresponding value is non-null (or coalesce to empty) so callers can safely pass null.

[Copilot]

Both SetMtouchUseLlvm and SetMtouchUseBitcode ignore the value parameter and always write "true" because they use true ? "true" : "false". This makes it impossible to set these properties to false. Use the value parameter when selecting the string to write.

[Copilot]

SimulatorLoader.SelectDevices return type was changed to IEnumerable<ISimulatorDevice>, but ISimulatorLoader.SelectDevices still declares IEnumerable<ISimulatorDevice?>. This will break the interface implementation (compile error) unless the interface signature is updated too (or the implementation keeps the nullable return type).