Fix Performance/CaseWhenSplat cop error on when node without body

Previously if `when` node has an empty body this cop raises:

```ruby
case 1
when 1 # no body
  # 1 == 1
else
  nil
end
```

```
undefined method `loc' for nil
    # ./lib/rubocop/cop/performance/case_when_splat.rb:154:in `indent_for'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:144:in `new_branch_without_then'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:123:in `reordering_correction'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:114:in `reorder_condition'
    # ./lib/rubocop/cop/performance/case_when_splat.rb:86:in `autocorrect'
```

It also did not preserve comments.

changelog/fix_performance_case_when_splat_cop_error_on_when_without_body.md

@@ -0,0 +1 @@
+* [#484](https://github.com/rubocop/rubocop-performance/pull/484): Fix `Performance/CaseWhenSplat` cop error on `when` node without body. ([@viralpraxis][])

lib/rubocop/cop/performance/case_when_splat.rb

@@ -58,6 +58,7 @@ module Performance
       class CaseWhenSplat < Base
         include Alignment
         include RangeHelp
+        include CommentsHelp
         extend AutoCorrector
 
         MSG = 'Reordering `when` conditions with a splat to the end of the `when` branches can improve performance.'
@@ -116,11 +117,18 @@ def reorder_condition(corrector, when_node)
         def reordering_correction(when_node)
           new_condition = replacement(when_node.conditions)
 
-          if same_line?(when_node, when_node.body)
-            new_condition_with_then(when_node, new_condition)
-          else
-            new_branch_without_then(when_node, new_condition)
-          end
+          condition =
+            if same_line?(when_node, when_node.body)
+              new_condition_with_then(when_node, new_condition)
+            else
+              new_branch_without_then(when_node, new_condition)
+            end
+
+          condition_comments = comments_in_range(when_node).map do |comment_node|
+            "#{indent_for(comment_node)}#{comment_node.source}"
+          end.join("\n")
+
+          "#{condition}#{condition_comments}"
         end
 
         def when_branch_range(when_node)
@@ -134,7 +142,13 @@ def new_condition_with_then(node, new_condition)
         end
 
         def new_branch_without_then(node, new_condition)
-          "\n#{indent_for(node)}when #{new_condition}\n#{indent_for(node.body)}#{node.body.source}"
+          new_branch = "\n#{indent_for(node)}when #{new_condition}\n"
+
+          if node.body
+            "#{new_branch}#{indent_for(node.body)}#{node.body.source}"
+          else
+            new_branch
+          end
         end
 
         def indent_for(node)

spec/rubocop/cop/performance/case_when_splat_spec.rb

@@ -54,6 +54,31 @@
     RUBY
   end
 
+  it 'registers an offense for case when with a splat in the first condition with branch without body' do
+    expect_offense(<<~RUBY)
+      case foo
+      when *cond
+      ^^^^^^^^^^ Reordering `when` conditions with a splat to the end of the `when` branches can improve performance.
+        # bar
+        # bar
+          # bar
+      when 3
+        baz
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      case foo
+      when 3
+        baz
+      when *cond
+        # bar
+        # bar
+          # bar
+      end
+    RUBY
+  end
+
   it 'registers an offense for case when with a splat without an else' do
     expect_offense(<<~RUBY)
       case foo