Disallow blank EppSession#session_id

#700

app/models/epp_session.rb

@@ -1,6 +1,8 @@
 class EppSession < ActiveRecord::Base
   before_save :marshal_data!
 
+  validates :session_id, presence: true
+
   belongs_to :registrar
   # rubocop: disable Rails/ReadWriteAttribute
   # Turned back to read_attribute, thus in Rails 4

test/models/epp_session_test.rb

@@ -1,8 +1,17 @@
 require 'test_helper'
 
 class EppSessionTest < ActiveSupport::TestCase
+  def setup
+    @epp_session = epp_sessions(:api_bestnames)
+  end
+
+  def test_valid
+    assert @epp_session.valid?
+  end
+
   def test_api_user_id_serialization
     epp_session = EppSession.new
+    epp_session.session_id = 'test'
     epp_session.registrar = registrars(:bestnames)
     epp_session[:api_user_id] = ActiveRecord::Fixtures.identify(:api_bestnames)
     epp_session.save!
@@ -10,4 +19,10 @@ class EppSessionTest < ActiveSupport::TestCase
 
     assert_equal ActiveRecord::Fixtures.identify(:api_bestnames), epp_session[:api_user_id]
   end
+
+  def test_session_id_presence_validation
+    @epp_session.session_id = nil
+    @epp_session.validate
+    assert @epp_session.invalid?
+  end
 end