Prefer empty relation rather than arrays

app/models/project.rb

@@ -431,7 +431,7 @@ class Project < ActiveRecord::Base
 
   def builds_for(build_name, ref = 'HEAD')
     ct = commit(ref)
-    return [] unless ct
+    return builds.none unless ct
 
     sha = commit(ref).sha
 

spec/models/build_spec.rb

@@ -697,5 +697,11 @@ describe Ci::Build, models: true do
 
       expect(build_ids).to eq([build.id])
     end
+
+    it 'returns empty relation if the build cannot be found' do
+      builds = project.builds_for(build.name, 'TAIL').all
+
+      expect(builds).to be_empty
+    end
   end
 end