perf: use Nullable<T>.GetValueOrDefault() instead of Nullable<T>.Value

[nogic1008]

Nullable<T>.Value always does null check as below.

If we know it's not null, we can avoid the extra if by using Nullable<T>.GetValueOrDefault() instead.

public struct Nullable<T> where T : struct
{
    private readonly bool hasValue;
    internal T value;

    public readonly T Value
    {
        get
        {
            if (!hasValue)
            {
                ThrowHelper.ThrowInvalidOperationException_InvalidOperation_NoValue();
            }
            return value;
        }
    }

    public readonly T GetValueOrDefault() => value;
}

Inspired by dotnet/coreclr#22297, .NET blog post

[neuecc]

Thank you, I wasn't aware of that at all!
Thank you also for the blog reference, it's very helpful!