Fix exception creating an answer without an author

We were getting `undefined method `lock!' for nil:NilClass` when the
question allowed multiple answers.

app/models/poll/answer.rb

@@ -36,7 +36,7 @@ class Poll::Answer < ApplicationRecord
   private
 
     def max_votes
-      return if !question || question&.unique? || persisted?
+      return if !question || !author || question&.unique? || persisted?
 
       author.lock!
 

spec/models/poll/answer_spec.rb

@@ -18,6 +18,13 @@ describe Poll::Answer do
       expect(answer).not_to be_valid
     end
 
+    it "is not valid without an author when multiple answers are allowed" do
+      answer.author = nil
+      answer.question = create(:poll_question_multiple)
+
+      expect(answer).not_to be_valid
+    end
+
     it "is not valid without an answer" do
       answer.answer = nil
       expect(answer).not_to be_valid