Use case instead of if

It reads nicer, and `flog` agrees. Before:

❯ flog lib/rubocop/cop/rspec/mixin/top_level_group.rb
    29.7: flog total
     5.0: flog/method average

     9.8: RuboCop::Cop::RSpec::TopLevelGroup#top_level_nodes lib/rubocop/cop/rspec/mixin/top_level_group.rb:36-44
     7.6: RuboCop::Cop::RSpec::TopLevelGroup#on_new_investigation lib/rubocop/cop/rspec/mixin/top_level_group.rb:10-16
     4.9: RuboCop::Cop::RSpec::TopLevelGroup#top_level_groups lib/rubocop/cop/rspec/mixin/top_level_group.rb:20-22

After:

❯ flog lib/rubocop/cop/rspec/mixin/top_level_group.rb
    25.8: flog total
     4.3: flog/method average

     7.2: RuboCop::Cop::RSpec::TopLevelGroup#top_level_nodes lib/rubocop/cop/rspec/mixin/top_level_group.rb:35-44
     6.3: RuboCop::Cop::RSpec::TopLevelGroup#on_new_investigation lib/rubocop/cop/rspec/mixin/top_level_group.rb:10-15
     4.9: RuboCop::Cop::RSpec::TopLevelGroup#top_level_groups lib/rubocop/cop/rspec/mixin/top_level_group.rb:19-21

Author: bquorning

lib/rubocop/cop/rspec/mixin/top_level_group.rb

@@ -9,7 +9,6 @@ module TopLevelGroup
 
         def on_new_investigation
           super
-          return unless root_node
 
           top_level_groups.each do |node|
             on_top_level_example_group(node) if example_group?(node)
@@ -34,11 +33,12 @@ def top_level_group?(node)
         end
 
         def top_level_nodes(node)
-          if node.nil?
-            []
-          elsif node.begin_type?
+          return [] if node.nil?
+
+          case node.type
+          when :begin
             node.children
-          elsif node.module_type? || node.class_type?
+          when :module, :class
             top_level_nodes(node.body)
           else
             [node]