try this
if @Clients_Title != NULL OR @Address_Current != NULL OR @Phone_Number != NULL OR @Mobile_Number !+ NULL OR @AreaLocation != NULL
Begin
SELECT Cust_Id, Clients_Title, Card_Number, Key_Person, Address_Current, Phone_Number, Mobile_Number, AreaLocation
FROM Customer_New
WHERE (Clients_Title = @Clients_Title) OR
(Address_Current = @Address_Current) OR
(Phone_Number = @Phone_Number) OR
(Mobile_Number = @Mobile_Number) OR
(AreaLocation = @AreaLocation)
else
SELECT Cust_Id, Clients_Title, Card_Number, Key_Person, Address_Current, Phone_Number, Mobile_Number, AreaLocation
FROM Customer_New
End