Simplify unnecessary condition

[Youssef1313]

I think the length should be guaranteed to be 1 in this code path since we check for AttributeDescription.CallerArgumentExpressionAttribute.

Note: I initially wrote it that way since it was nearly a copy/paste from other code here:

roslyn/src/Compilers/CSharp/Portable/Symbols/Source/SourceComplexParameterSymbol.cs

Lines 776 to 782 in 1f5c03e

	private static bool? DecodeMaybeNullWhenOrNotNullWhenOrDoesNotReturnIfAttribute(AttributeDescription description, CSharpAttributeData attribute)
	{
	var arguments = attribute.CommonConstructorArguments;
	return arguments.Length == 1 && arguments[0].TryDecodeValue(SpecialType.System_Boolean, out bool value) ?
	(bool?)value :
	null;
	}

If my thought is correct, the other code could be simplified as well.

Test plan: #52745

[333fred]

@dotnet/roslyn-compiler for a second small review.

[jcouv]

LGTM Thanks (iteration 2)

[jcouv]

Merged/squashed. Thanks @Youssef1313
Also added a link to test plan #52745