FIX: Job exception: undefined method `email' for nil:NilClass

It seems that due to jobs being asynchronous and wrapping code in a
DistributedMutex that by the time we run the
`UserAvatar#update_gravatar!` job that the user/user email might be
destroyed.

This patch checks before a call to `user.email_hash` to make sure
the user and primary email exist to prevent the exception. If not
present, the job exits as there's nothing to do because we are
probably running after the user was destroyed for some reason.

app/models/user_avatar.rb

@@ -16,6 +16,10 @@ class UserAvatar < ActiveRecord::Base
         self.update!(last_gravatar_download_attempt: Time.now)
 
         max = Discourse.avatar_sizes.max
+
+        # The user could be deleted before this executes
+        return if user.blank? || user.primary_email.blank?
+
         email_hash = user_id == Discourse::SYSTEM_USER_ID ? User.email_hash("info@discourse.org") : user.email_hash
         gravatar_url = "https://www.gravatar.com/avatar/#{email_hash}.png?s=#{max}&d=404"
 

spec/models/user_avatar_spec.rb

@@ -106,6 +106,13 @@ describe UserAvatar do
       end
     end
 
+    it "should not raise an error when there's no primary_email" do
+      avatar.user.primary_email.destroy
+      avatar.user.reload
+
+      # If raises an error, test fails
+      avatar.update_gravatar!
+    end
   end
 
   context '.import_url_for_user' do