Changed ActiveRecord migration's column defaults

Hey there!

I changed the defaults of the "amount" column of ActiveRecord migrations so that the column will allow null values and so that it no longer has a default value of 0.

The amount column should not have such settings by default because there are plenty of use cases where one should be able to store an empty value. This is consistent with ActiveRecord defaults...if somebody wants to make a column mandatory then they should specifically specify `:null: false`, and by the same token they should specify their own default values.

Apologies if this isn't clear. I'm a bit tired...