Previously, this migration would cause the error: "Cannot add a NOT NULL
column with default value NULL". This fixes that by setting the default
value for new columns to '' (empty string). It updates the query builder
to support this, too.
Combining all operations into a single query was causing problems in
SQLite. This fixes that by breaking them up into separate queries. It
also moves one column length change to only run on MySQL, since SQLite
doesn't need it.
It's not straightforward to remove these constraints in SQLite, so this
just skips it entirely. Since both of these migrations are part of the
same WF release, this should have minimal impact on admins.
