Enable nullable reference types in AndroidDiskInformation.cs

[Copilot]

src/Mono.AndroidTools/AndroidDiskInformation.cs ships in the signed Mono.AndroidTools assembly but was nullable-oblivious. This opts the file into nullable reference types for compile-time null-safety with no runtime or API change.

Changes

• Added #nullable enable to the top of the file.
• Changed the s locals in ParseNewFormat and ParseOldFormat from string to string?, matching StringReader.ReadLine()'s string? return. Flow analysis already narrows s to non-null inside the while ((s = reader.ReadLine ()) != null) bodies.

string? s;
var reader = new StringReader (output);
while ((s = reader.ReadLine ()) != null) {
    // s is non-null here
}

Annotation-only: no ! operator, no ArgumentNullException.ThrowIfNull (the project targets netstandard2.0), and public signatures (FromDfOutput, InternalSpace, ExternalSpace) are unchanged.

[Copilot]

Pull request overview

This PR opts AndroidDiskInformation.cs into nullable reference types to improve compile-time null safety in the signed Mono.AndroidTools assembly, without changing runtime behavior.

Changes:

• Enabled nullable annotations via #nullable enable.
• Updated local variables that receive StringReader.ReadLine() results from string to string? in the parsing helpers.

Show a summary per file

File	Description
src/Mono.AndroidTools/AndroidDiskInformation.cs	Enables NRT and aligns ReadLine() locals with string? to avoid nullable warnings and improve flow analysis.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1