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.

plugins/include/functions.inc

@@ -290,6 +290,16 @@ native void Call_PushArray(const any[] value, int size);
  */
 native void Call_PushArrayEx(any[] value, int size, int cpflags);
 
+/**
+ * Pushes the NULL_VECTOR onto the current call.
+ * @see IsNullVector
+ *
+ * @note Cannot be used before a call has been started.
+ *
+ * @error					Called before a call has been started.
+ */
+native void Call_PushNullVector();
+
 /**
  * Pushes a string onto the current call.
  *
@@ -317,6 +327,16 @@ native void Call_PushString(const char[] value);
  */
 native void Call_PushStringEx(char[] value, int length, int szflags, int cpflags);
 
+/**
+ * Pushes the NULL_STRING onto the current call.
+ * @see IsNullString
+ *
+ * @note Cannot be used before a call has been started.
+ *
+ * @error					Called before a call has been started.
+ */
+native void Call_PushNullString();
+
 /**
  * Completes a call to a function or forward's call list.
  *