Introduce `RandomOrDefault` extension method

[Anders Bjerner]

Eg:

/// <summary>
/// Returns a random item from the specified <paramref name="collection"/>.
/// </summary>
/// <typeparam name="T">The type the items.</typeparam>
/// <param name="collection">The collection.</param>
/// <returns>A random item of type <typeparamref name="T"/> from <paramref name="collection"/> if not empty; otherwise, the default value of <typeparamref name="T"/>.</returns>
public static T RandomOrDefault<T>(this IEnumerable<T> collection) {
    return collection.OrderBy(x => Guid.NewGuid()).FirstOrDefault();
}