为什么数组中的前导null值会导致string.Join（）返回空字符串？

Question

以下测试按书面要求通过。为什么？（即，为什么数组中的前导null值导致string.Join（）返回空字符串？）

[TestMethod]

public void Test_string_Join()

{

    object[] values0 = { "foo", 3, null };

    var text0 = string.Join(", ", values0);

    Console.WriteLine("text0 = " + text0);

    AssertX.AreEqual("foo, 3, ", text0); // works as expected

    object[] values1 = { null, "foo", 3, null };

    var text1 = string.Join(", ", values1);

    Console.WriteLine("text1 = " + text1);

    AssertX.AreEqual("", text1); // does NOT work as expected, why empty?

}