Add nullable to "last_name" and "gender" columns in customer migration

[iagobruno]

For Postgres don't complain about non-null columns.

[gabolera]

this PR solve 2 bugs on execute php artisan shopper:admin because columns "first_name" and "gender" that not defined with nullable

is this has identical discussion in #104

@mckenziearts please analyse this PR

thanks :)

[mckenziearts]

This PR cannot be valid. You had to write a new migration to fix this problem, and moreover the first_name field is nullable by default so it is last_name that I use in my views so it cannot be nullable.

Those who have already installed and used shopper will not have this modification applied since their site will already be online and the project already functional

And for the gender I will publish an Enum to avoid the problem with postgre in the next version.

If you want to correct or propose a solution you can make a PR that is structured this way. I'll close it until I get another proposal