Don't break backwards compatibility with unmanaged forwards

Instead of adding new functions to `IForward`, we just allow NULL to be passed to `PushArray` and `PushString`.

core/logic/ForwardSys.h

@@ -55,8 +55,6 @@ public: //IForward
 	virtual unsigned int GetFunctionCount();
 	virtual ExecType GetExecType();
 	virtual int Execute(cell_t *result, IForwardFilter *filter);
-	virtual int PushNullString();
-	virtual int PushNullVector();
 public: //IChangeableForward
 	virtual bool RemoveFunction(IPluginFunction *func);
 	virtual unsigned int RemoveFunctionsOfPlugin(IPlugin *plugin);
@@ -74,6 +72,8 @@ private:
 	CForward(ExecType et, const char *name,
 	         const ParamType *types, unsigned num_params);
 
+	int PushNullString();
+	int PushNullVector();
 	int _ExecutePushRef(IPluginFunction *func, ParamType type, FwdParamInfo *param);
 	void _Int_PushArray(cell_t *inarray, unsigned int cells, int flags);
 	void _Int_PushString(cell_t *inarray, unsigned int cells, int sz_flags, int cp_flags);