C#: Harden cs/useless-cast-to-self for BMN extracted databases.

Author: michaelnebel

csharp/ql/src/Language Abuse/UselessCastToSelf.ql

@@ -19,5 +19,6 @@ where
   type = cast.getTargetType() and
   type = e.getType() and
   not type instanceof NullType and
+  not type instanceof UnknownType and
   not e.(ImplicitDelegateCreation).getArgument() instanceof AnonymousFunctionExpr
 select cast, "This cast is redundant because the expression already has type " + type + "."