ensure fork_source returns nil when project is the root fork, add tests

app/models/project.rb

@@ -1032,6 +1032,8 @@ class Project < ActiveRecord::Base
   end
   def fork_source
+    return nil unless forked?
+
     forked_from_project || fork_network&.root_project
   end

spec/models/project_spec.rb

@@ -1951,6 +1951,10 @@ describe Project do
         expect(second_fork.fork_source).to eq(project)
       end
+
+      it 'returns nil if it is the root of the fork network' do
+        expect(project.fork_source).to be_nil
+      end
     end
     describe '#lfs_storage_project' do