Enable nullable reference types in RtxtWriter.cs

[Copilot]

Opts RtxtWriter.cs into NRT as part of the repo-wide nullable enablement effort.

Changes

• Added #nullable enable as the first line of RtxtWriter.cs
• Added explicit null guards for the file and items parameters using netstandard2.0-compatible form (ArgumentNullException.ThrowIfNull is unavailable on that TFM):

if (file == null)
    throw new ArgumentNullException (nameof (file));
if (items == null)
    throw new ArgumentNullException (nameof (items));

R is a struct, so IList<R> requires no element-level null annotations and no ! operator is needed anywhere.

• Fixes [fix-finder] Enable nullable reference types in RtxtWriter.cs #12240

[Copilot]

Pull request overview

This PR opts src/Xamarin.Android.Build.Tasks/Utilities/RtxtWriter.cs into nullable reference types (NRT) as part of the repo-wide enablement effort, adding explicit runtime guards for reference-type inputs to preserve behavior and eliminate nullable warnings.

Changes:

• Enabled NRT with #nullable enable at the top of RtxtWriter.cs.
• Added ArgumentNullException guards for file and items using a netstandard2.0-compatible pattern.