Localize sell and buy only messages

PrimaryWeapon always returns "", never nil unless the player object
isNull (in which case this script would not be running).

Fixed cases where a nil variable would throw an error when checking the
next condition.

SQF/dayz_code/actions/AdvancedTrading/init.sqf

@@ -2,7 +2,7 @@ disableSerialization;
 
 Z_traderData = (_this select 3); // gets the trader data ( e.g menu_Functionary1 )
 
-if (isNil "Z_traderData" || count (Z_traderData) == 0) exitWith {
+if (isNil "Z_traderData" or {count Z_traderData == 0}) exitWith {
 	localize "STR_EPOCH_TRADE_ERROR" call dayz_rollingMessages;
 };