Add missing nullability annotation to string ctor

[GrabYourPitchforks]

The constructor string(char[] value) allows null as an input. We forgot to put the ? annotation on the parameter. There's already a unit test validating that we allow null inputs, as shown below.

runtime/src/libraries/Common/tests/Tests/System/StringTests.cs

Lines 150 to 157 in b9f6f5f

	[InlineData(null, 0, 0, "")]
	public static void Ctor_CharArray(char[] value, int startIndex, int length, string expected)
	{
	if (value == null)
	{
	Assert.Equal(expected, new string(value));
	return;
	}

[stephentoub]

Thanks.

[jkotas]

The CI failures are know issue #2176