Allow blank votes in polls via web

With the old interface, there wasn't a clear way to send a blank ballot.
But now that we've got a form, there's an easy way: clicking on "Vote"
while leaving the form blank.

app/models/poll.rb

@@ -23,6 +23,7 @@ class Poll < ApplicationRecord
   has_many :officer_assignments, through: :booth_assignments
   has_many :officers, through: :officer_assignments
   has_many :questions, inverse_of: :poll, dependent: :destroy
+  has_many :answers, through: :questions
   has_many :comments, as: :commentable, inverse_of: :commentable
   has_many :ballot_sheets
 

app/models/poll/stats.rb

@@ -42,11 +42,11 @@ class Poll::Stats
   end
 
   def total_web_valid
-    voters.where(origin: "web").count - total_web_white
+    voters.where(origin: "web", user_id: poll.answers.select(:author_id).distinct).count
   end
 
   def total_web_white
-    0
+    voters.where(origin: "web").count - total_web_valid
   end
 
   def total_web_null

app/models/poll/web_vote.rb

@@ -14,10 +14,6 @@ class Poll::WebVote
     all_valid = true
 
     user.with_lock do
-      unless questions.any? { |question| params.dig(question.id.to_s, :option_id).present? }
-        Poll::Voter.find_by(user: user, poll: poll, origin: "web")&.destroy!
-      end
-
       questions.each do |question|
         question.answers.where(author: user).destroy_all
         next unless params[question.id.to_s]