Apply Rails/FindBy rubocop rule

We were already using it in most places.

app/models/poll/ballot.rb

@@ -22,11 +22,11 @@ class Poll::Ballot < ApplicationRecord
   end
 
   def ballot
-    Budget::Ballot.where(poll_ballot: self).first
+    Budget::Ballot.find_by(poll_ballot: self)
   end
 
   def find_investment(investment_id)
-    ballot.budget.investments.where(id: investment_id).first
+    ballot.budget.investments.find_by(id: investment_id)
   end
 
   def not_already_added?(investment)

app/models/poll/booth.rb

@@ -23,7 +23,7 @@ class Poll
     end
 
     def assignment_on_poll(poll)
-      booth_assignments.where(poll: poll).first
+      booth_assignments.find_by(poll: poll)
     end
   end
 end

app/models/poll/voter.rb

@@ -56,7 +56,7 @@ class Poll
       def fill_stats_fields
         if in_census?
           self.gender = census_api_response.gender
-          self.geozone_id = Geozone.select(:id).where(census_code: census_api_response.district_code).first&.id
+          self.geozone_id = Geozone.select(:id).find_by(census_code: census_api_response.district_code)&.id
           self.age = voter_age(census_api_response.date_of_birth)
         end
       end