add improvements to the code

[wolfrednicolas]

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[wolfrednicolas]

@joshtorres

here are the changes of #45 and #43

since the amount is not null in the database, if by mistake the user leaves empty, automaticly fills with 0 (ZERO).. I think the best approach is create the rule required and if its empty show the message cannot be null.... I think this will improve the user performance