Nullable attributes don't modify indexer parameter initial nullable state

[RikkiGibson]

Version Used: fca6e1f

Steps to Reproduce:

[Fact]
public void AllowNull_Indexer()
{
    var source = """
        #nullable enable

        using System;
        using System.Diagnostics.CodeAnalysis;

        class C
        {
            public static void Main()
            {
                var c = new C();
                try
                {
                    _ = c[null]; // no warning
                }
                catch
                {
                    Console.Write(1);
                }
            }

            public string this[[AllowNull] string s]
            {
                get
                {
                    return s.ToString(); // expected a warning
                }
            }
        }
        """;

    var verifier = CompileAndVerify([source, AllowNullAttributeDefinition], expectedOutput: "1");
    verifier.VerifyDiagnostics();
}

Expected Behavior: A warning is reported on the indicated line. A warning would be reported for a similar scenario with an ordinary method.

Actual Behavior: No warning is reported in the sample