Add IEnumerable<T>.IsNullOrEmpty() extension method

[roman-yagodin]

Basic implementation:

using System.Collections.Generic;
using System.Linq;

namespace R7.Dnn.UserHtml.Models
{
    public static class EnumerableExtensions
    {
        public static bool IsNullOrEmpty<T> (this IEnumerable<T> source) =>
           source == null || !source.Any ();
    }
}

More advanced implementation would contain ICollection<T>.Count checks for better performance. Example: http://blog.vkuznetsov.ru/posts/2011/08/31/csharp-ienumerable-isnullorempty