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/go_inout_runme.go

@@ -50,4 +50,8 @@ func main() {
 		fmt.Println("for c2.M got", pm, "want", want)
 		panic(pm)
 	}
+
+	c1 := go_inout.NewC1()
+	c2.M2(c1)
+	c2.M2(nil)
 }

Examples/test-suite/go_inout.i

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