AddIfNotNull collection extensions

I want to post a few recently created collection extensions to write in one line, what otherwise takes 2 or more
     public static void AddIfNotNull( this IList coll, T newItem) where T : class
        {
            if (newItem != null)
            {
                coll.Add(newItem);
            }
        }

         public static void AddRangeIfNotNullOrEmpty( this List coll, IEnumerable newItems) where T : class
        {
            if (!newItems.IsNullOrEmptySequence())
            {
                coll.AddRange(newItems);
            }
        }
 
  public static void AddIfNotContains( this Dictionary dictionary,  TKey key, TValue value)
        {
            if (!dictionary.ContainsKey(key))
            {
                dictionary.Add(key, value);
            }
        }
The methods use
public static bool IsNullOrEmptySequence(this IEnumerable c)
              {
                      return (c == null || !c.Any() );
              }

I’ve also found a few extensions, that could be useful in https://pikacode.com/Barankin/Fabrika-dveri/file/default/CRM/Common/Extensions/LinqExtensions.cs