Read old nil C4Values from player files for compatibility with 5.1

src/script/C4Value.cpp

@@ -244,6 +244,7 @@ static C4V_Type GetC4VFromID(const char C4VID)
 	switch (C4VID)
 	{
 	case 'n':
+	case 'A': // compat with OC 5.1
 		return C4V_Nil;
 	case 'i':
 		return C4V_Int;