Full outer join will give NULL values for Table1(T1): CUSTOMER if we did not get data in Table2(T2): SALES_ORDER for that customer id.In simple way, you are getting NULL values for Customer Id = C4 because table2 does not have rows with customer id C4.
Same thing for Table2(T2): SALES_ORDER,you are getting NULL values for customer ID-5.
If you want all customer id should be display in final table then use inner join.Because inner join does not contains null values.And parent table should have all customer id.
Note:You can make foreign key relationship with Table1 and Table2 tables.So,at last both tables have same customer id.and then use inner join.You will never get null values.