[xharness] Fix finding testable types in test assemblies. Fixes #xama…

…rin/maccore@2267. (#9103)

Apparently a null namespace when using Reflection can be an empty namespace in Cecil.

Fixes xamarin/maccore#2267.

tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/TestImporter/ProjectDefinition.cs

@@ -127,7 +127,7 @@ public bool Validate ()
 					if (t.HasGenericParameters)
 						return false;
 
-					if (t.Namespace == null)
+					if (string.IsNullOrEmpty (t.Namespace))
 						return false;
 
 					if (!t.FullName.EndsWith ("Test", StringComparison.OrdinalIgnoreCase) && !t.FullName.EndsWith ("Tests", StringComparison.OrdinalIgnoreCase))