swig -go: treat a nil argument as NULL

Let Go code pass "nil" when calling a C++ function that accepts a
pointer to a class.  The Go "nil" will be treated as a C++ "nullptr".

Fixes #2203

Examples/test-suite/go_inout.i

@@ -239,5 +239,7 @@ class C1 {
 };
 
 class C2 : public C1 {
+ public:
+  void M2(C1*) {}
 };
 %}