NRE in `GetDiagnosticMessage`

[michael-sterling]

To Repro:

[Test]
public void TestTupleContext()
{
	using (AssertEx.Context("test", (Foo: "foo", Bar: "bar")))
		AssertEx.IsTrue(() => 1 == 0);
}

Expected:

Expected: 1 == 0
Context:
    test = ("foo", "bar")

Actual:

System.NullReferenceException : Object reference not set to an instance of an object.
   at Faithlife.Testing.AssertEx.<ToString>g__Property|24_0(String name, <>c__DisplayClass24_0& ) in D:\a\FaithlifeTesting\FaithlifeTesting\src\Faithlife.Testing\AssertEx.cs:line 411
   at Faithlife.Testing.AssertEx.ToString(Object obj) in D:\a\FaithlifeTesting\FaithlifeTesting\src\Faithlife.Testing\AssertEx.cs:line 397
   at Faithlife.Testing.AssertEx.GetDiagnosticMessage(String expected, IReadOnlyCollection`1 actualValues, Exception e, IEnumerable`1 immediateContextValues) in D:\a\FaithlifeTesting\FaithlifeTesting\src\Faithlife.Testing\AssertEx.cs:line 339
   at Faithlife.Testing.AssertEx.GetDiagnosticMessage(Expression body, Exception e, IEnumerable`1 immediateContextValues) in D:\a\FaithlifeTesting\FaithlifeTesting\src\Faithlife.Testing\AssertEx.cs:line 312
   at Faithlife.Testing.AssertEx.IsTrue(Expression`1 predicateExpression) in D:\a\FaithlifeTesting\FaithlifeTesting\src\Faithlife.Testing\AssertEx.cs:line 29