Don't generate dbml refs for embedded schemas

[DReigada]

Currently for embedded schemas that reference other schemas, the generated dbml file is invalid.

Example:

This schema

typed_schema "my_schema" do
  embeds_many :embedded_schema, EmbeddedSchema, on_replace: :delete, primary_key: false do
    field(:my_field, :string)

    belongs_to(:other_schema, OtherSchema, primary_key: true, type: :binary_id)
  end
end

Generate a Ref "line" like the following (notice the empty string):

Ref: other_schema.id < "".other_schema_id

This PR changes the behaviour to ignore lines like this.

[fuelen]

Thank you!