Update minimum 421 AI fixed size to 3
Update minimum 421 AI fixed size to 3

The referenced rule given in below link shows ^421(\d{3})([\x21-\x22\x25-\x2F\x30-\x39\x3A-\x3F\x41-\x5A\x5F\x61-\x7A]{0,9})$ 
This indicates a minimum of 3 decimal.
https://www.gs1.org/standards/barcodes/application-identifiers/421?lang=en
woo-j/OkapiBarcode