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?}