Don't break when building unions on empty collections

lib/gitlab/sql/union.rb

@@ -26,7 +26,11 @@ module Gitlab
           @relations.map { |rel| rel.reorder(nil).to_sql }.reject(&:blank?)
         end
 
+        if fragments.any?
           fragments.join("\n#{union_keyword}\n")
+        else
+          'NULL'
+        end
       end
 
       def union_keyword

spec/lib/gitlab/sql/union_spec.rb

@@ -29,5 +29,12 @@ describe Gitlab::SQL::Union do
 
       expect(union.to_sql).to include('UNION ALL')
     end
+
+    it 'returns `NULL` if all relations are empty' do
+      empty_relation = User.none
+      union = described_class.new([empty_relation, empty_relation])
+
+      expect(union.to_sql).to eq('NULL')
+    end
   end
 end