Skip data_schema.rb dump when no data migrations are pending

Running `rails data:migrate` updates `db/data_schema.rb` even when there are no pending data migrations. This creates unnecessary file changes in version control.

## Request

Only dump db/data_schema.rb when data migrations actually run.

Related: #182 

I'm using version 9.4.2. Upgrading isn't an option at this time as we're using Rails LTS