Migration generator creates schema with blank schema name

[philayres]

During testing, a CreateSchema... migration is being created with an empty schema string:

require 'active_record/migration/app_generator'
class CreateSchema1ragas2bzo < ActiveRecord::Migration[5.2]
  include ActiveRecord::Migration::AppGenerator

  def change
    self.schema = ''
    self.owner = 'fphs'
    create_schema
  end
end

We should never request the migration generator to actually create a schema that has no name, or if this is not feasible, the migration generator should just log and skip the generation.