Handle nil and blank cases in Account finders (#3500)

app/models/concerns/account_finder_concern.rb

@@ -37,21 +37,25 @@ module AccountFinderConcern
 
     def scoped_accounts
       Account.unscoped.tap do |scope|
+        scope.merge! with_usernames
         scope.merge! matching_username
         scope.merge! matching_domain
       end
     end
 
+    def with_usernames
+      Account.where.not(username: [nil, ''])
+    end
+
     def matching_username
-      raise(ActiveRecord::RecordNotFound) if username.blank?
-      Account.where(Account.arel_table[:username].lower.eq username.downcase)
+      Account.where(Account.arel_table[:username].lower.eq username.to_s.downcase)
     end
 
     def matching_domain
       if domain.nil?
         Account.where(domain: nil)
       else
-        Account.where(Account.arel_table[:domain].lower.eq domain.downcase)
+        Account.where(Account.arel_table[:domain].lower.eq domain.to_s.downcase)
       end
     end
   end