Avoid empty Arrays in ActiveSupport::Callbacks

ActiveSupport::Callbacks often ended up with empty Arrays: on callbacks
without a conditional, and on callback sequences which had no before
and/or after callbacks.

activesupport/lib/active_support/callbacks.rb

@@ -323,8 +323,10 @@ def check_conditionals(conditionals)
           end
 
           def conditions_lambdas
-            @if.map { |c| CallTemplate.build(c, self).make_lambda } +
+            conditions =
+              @if.map { |c| CallTemplate.build(c, self).make_lambda } +
               @unless.map { |c| CallTemplate.build(c, self).inverted_lambda }
+            conditions.empty? ? EMPTY_ARRAY : conditions
           end
       end
 
@@ -517,16 +519,18 @@ def initialize(nested = nil, call_template = nil, user_conditions = nil)
           @call_template = call_template
           @user_conditions = user_conditions
 
-          @before = []
-          @after = []
+          @before = nil
+          @after = nil
         end
 
         def before(before)
+          @before ||= []
           @before.unshift(before)
           self
         end
 
         def after(after)
+          @after ||= []
           @after.push(after)
           self
         end
@@ -550,11 +554,11 @@ def expand_call_template(arg, block)
         end
 
         def invoke_before(arg)
-          @before.each { |b| b.call(arg) }
+          @before&.each { |b| b.call(arg) }
         end
 
         def invoke_after(arg)
-          @after.each { |a| a.call(arg) }
+          @after&.each { |a| a.call(arg) }
         end
       end