Remove the word null from blank fields [#62240086]

This pull request is to avoid showing the null word in the trade/shipments table.

It consists in the insertion of new codes for blank values in  the trade_codes tables and a change in the trade_shipments_view to use those codes and get an empty string instead of a null value.

You will need firstly to run:

rake import:trade_codes_null

Then 

rake db:migrate

Then you should be able to see empty strings instead of the old null values