Exclude nil balloters from vote phase participants

The `user_id` can be `nil` for ballots.

app/models/budget/stats.rb

@@ -65,7 +65,7 @@ class Budget::Stats
     end
 
     def balloters
-      budget.ballots.where("ballot_lines_count > ?", 0).pluck(:user_id)
+      budget.ballots.where("ballot_lines_count > ?", 0).pluck(:user_id).compact
     end
 
     def poll_ballot_voters

spec/models/budget/stats_spec.rb

@@ -69,6 +69,14 @@ describe Budget::Stats do
 
       expect(stats.generate[:total_participants_vote_phase]).to be 1
     end
+
+    it "doesn't count nil user ids" do
+      create(:budget_ballot_line, investment: investment,
+        ballot: create(:budget_ballot, budget: budget, user: nil, physical: true)
+      )
+
+      expect(stats.generate[:total_participants_vote_phase]).to be 0
+    end
   end
 
   describe "#total_participants_web" do