Fix error in user warning template (mastodon#11741)

Author: Gargron

app/views/user_mailer/warning.html.haml

@@ -42,11 +42,11 @@
                               - unless @warning.text.blank?
                                 = Formatter.instance.linkify(@warning.text)
 
-                              - unless @statuses&.empty?
+                              - if !@statuses.nil? && !@statuses.empty?
                                 %p
                                   %strong= t('user_mailer.warning.statuses')
 
-- unless @statuses&.empty?
+- if !@statuses.nil? && !@statuses.empty?
   - @statuses.each_with_index do |status, i|
     = render 'notification_mailer/status', status: status, i: i + 1, highlighted: true
 

app/views/user_mailer/warning.text.erb

@@ -7,7 +7,7 @@
 
 <% end %>
 <%= @warning.text %>
-<% unless @statuses&.empty? %>
+<% if !@statuses.nil? && !@statuses.empty? %>
 <%= t('user_mailer.warning.statuses') %>
 
 <% @statuses.each do |status| %>