Migrations with milestones raise error if initialized without constructor parameters

Background

We started requiring milestones in !133865 (merged). Most of the time, migrations are initialized with parameters passed to the constructor, but I discovered through a spec test in an unrelated branch that if the constructor is passed with no parameters, the default first parameter is class_name, which is undefined.

Steps to recreate

1. Generate a version 2.2 migration
2. In the rails console, call new on the migration class you just created

Assigning a lower severity since we have until November 1 to add keep adding version 2.1 migrations and the fix is trivial.