Add Call_PushNullVector and Call_PushNullString

Be able to push NULL_[VECTOR|STRING] to a forward or direct function call.
The callee can check the parameter using the IsNullVector/IsNullString natives.

public/IForwardSys.h

@@ -50,7 +50,7 @@
 using namespace SourcePawn;
 
 #define SMINTERFACE_FORWARDMANAGER_NAME		"IForwardManager"
-#define SMINTERFACE_FORWARDMANAGER_VERSION	3
+#define SMINTERFACE_FORWARDMANAGER_VERSION	4
 
 /*
  * There is some very important documentation at the bottom of this file.
@@ -118,6 +118,7 @@ namespace SourceMod
 		cell_t val;
 		ByrefInfo byref;
 		ParamType pushedas;
+		bool isnull;
 	};
 	
 	class IForwardFilter
@@ -183,6 +184,22 @@ namespace SourceMod
 		 * @return			Error code, if any.
 		 */
 		virtual int PushArray(cell_t *inarray, unsigned int cells, int flags=0) =0;
+
+		/**
+		 * @brief Pushes the NULL_STRING onto the current call. This will always push the
+		 * correct reference to each function in the forward.
+		 *
+		 * @return			Error code, if any.
+		 */
+		virtual int PushNullString() =0;
+
+		/**
+		 * @brief Pushes the NULL_VECTOR onto the current call. This will always push the
+		 * correct reference to each function in the forward.
+		 *
+		 * @return			Error code, if any.
+		 */
+		virtual int PushNullVector() =0;
 	};
 
 	/**