Add and apply Style/RedundantCondition rule

This is a rule we generally follow all the time but accidentally forgot
in one place.

.rubocop.yml

@@ -455,6 +455,9 @@ Style/Proc:
 Style/RaiseArgs:
   Enabled: true
 
+Style/RedundantCondition:
+  Enabled: true
+
 Style/RedundantFileExtensionInRequire:
   Enabled: true
 

app/helpers/welcome_helper.rb

@@ -34,7 +34,7 @@ module WelcomeHelper
   def calculate_carousel_size(debates, proposals, apply_offset)
     offset = calculate_offset(debates, proposals, apply_offset)
     centered = calculate_centered(debates, proposals)
-    "#{offset if offset} #{centered if centered}"
+    "#{offset} #{centered}"
   end
 
   def calculate_centered(debates, proposals)