[Enhancement] DefaultIfEmptyF should be implemented

[Simon Coghlan]

Both versions of this to work with List<T>, Array[T], Span<T>, ReadOnlySpan<T> and IReadOnlyList<T>

If the source is null or empty, then default to creating an array so that max speed is maintained.

[Simon Coghlan]

Should this also follow the pattern for the other "Fast" api's e.g WhereSelect
i.e.

public static int MaxOrDefault<T>(this IEnumerable<T> enumeration, Func<T, int> selector)
{
    return enumeration.Any() ? enumeration.Max(selector) : default(int);
}