Fix Error 500 resulting when loading network graph

`discussion_id` may not be present when the SELECT call for notes
does not include this attribute. Don't attempt to set the discussion ID
unless the model contains the attribute.

Closes #21119, #21128

app/models/note.rb

@@ -259,6 +259,8 @@ class Note < ActiveRecord::Base
 
   def ensure_discussion_id
     return unless self.persisted?
+    # Needed in case the SELECT statement doesn't ask for `discussion_id`
+    return unless self.has_attribute?(:discussion_id)
     return if self.discussion_id
 
     set_discussion_id

spec/models/network/graph_spec.rb

+require 'spec_helper'
+
+describe Network::Graph, models: true do
+  let(:project) { create(:project) }
+  let!(:note_on_commit) { create(:note_on_commit, project: project) }
+
+  it '#initialize' do
+    graph = described_class.new(project, 'refs/heads/master', project.repository.commit, nil)
+
+    expect(graph.notes).to eq( { note_on_commit.commit_id => 1 } )
+  end
+end