fix ConstantAttributeAnalyzer null reference

[omsmith]

Fixes a NullReferenceException during analysis of an InterpolatedStringHandler usage.

Possible to add a test, but requires a couple of package updates that I don't want to verify are safe, at the moment.

[InterpolatedStringHandler]
public sealed class MyString {
  public MyString( int a, int b ) {}
  public void AppendLiteral( [Constant] string s ) {}
  public void AppendFormatted( [Constant] string s ) {}
}

public sealed class C {
  public void M( bool x ) {

    R(
      $"""
      FOO
      {( x ? "x" : "y" )}
      """
    );
  }

  public void R( MyString s ) {}
}

This desugars to

  public void M( bool x ) {
    MyString <> = new MyString( 4, 1 );
    <>.AppendLiteral( "FOO\n" );
    <>.AppendFormatted( x ? "x" : "y" );
    R( <> );
  }

However, the syntax for the operation of <>.AppendFormatted( x ? "x" : "y" ); is still from the original interpolated string; both argument.Syntax and argument.Value.Syntax are ConditionalExpressionSyntax rather than an ArgumentSyntax.

SPLA-3693

[j3parker]

lol nice