Add new Style/EmptyHeredoc cop

## Summary

Checks for using empty heredoc to reduce redundancy.

```ruby
# bad
<<~EOS
EOS

<<-EOS
EOS

<<EOS
EOS

# good
''

# bad
do_something(<<~EOS)
EOS

do_something(<<-EOS)
EOS

do_something(<<EOS)
EOS

# good
do_something('')
```

## Other Information

I noticed the following redundancy in the RuboCop test code:

```ruby
expect_correction(<<~RUBY)
RUBY
```

This cop solves it generically.

changelog/new_add_new_style_empty_heredoc_cop.md

@@ -0,0 +1 @@
+* [#10820](https://github.com/rubocop/rubocop/pull/10820): Add new `Style/EmptyHeredoc` cop. ([@koic][])

config/default.yml

@@ -3514,6 +3514,11 @@ Style/EmptyElse:
     - both
   AllowComments: false
 
+Style/EmptyHeredoc:
+  Description: 'Checks for using empty heredoc to reduce redundancy.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Style/EmptyLambdaParameter:
   Description: 'Omit parens for empty lambda parameters.'
   Enabled: true

lib/rubocop.rb

@@ -481,6 +481,7 @@
 require_relative 'rubocop/cop/style/empty_block_parameter'
 require_relative 'rubocop/cop/style/empty_case_condition'
 require_relative 'rubocop/cop/style/empty_else'
+require_relative 'rubocop/cop/style/empty_heredoc'
 require_relative 'rubocop/cop/style/empty_lambda_parameter'
 require_relative 'rubocop/cop/style/empty_literal'
 require_relative 'rubocop/cop/style/empty_method'

lib/rubocop/cop/style/empty_heredoc.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # Checks for using empty heredoc to reduce redundancy.
+      #
+      # @example
+      #
+      #   # bad
+      #   <<~EOS
+      #   EOS
+      #
+      #   <<-EOS
+      #   EOS
+      #
+      #   <<EOS
+      #   EOS
+      #
+      #   # good
+      #   ''
+      #
+      #   # bad
+      #   do_something(<<~EOS)
+      #   EOS
+      #
+      #   do_something(<<-EOS)
+      #   EOS
+      #
+      #   do_something(<<EOS)
+      #   EOS
+      #
+      #   # good
+      #   do_something('')
+      #
+      class EmptyHeredoc < Base
+        include Heredoc
+        include RangeHelp
+        extend AutoCorrector
+
+        MSG = 'Use an empty string literal instead of heredoc.'
+
+        def on_heredoc(node)
+          heredoc_body = node.loc.heredoc_body
+
+          return unless heredoc_body.source.empty?
+
+          add_offense(node) do |corrector|
+            heredoc_end = node.loc.heredoc_end
+
+            corrector.replace(node, "''")
+            corrector.remove(range_by_whole_lines(heredoc_body, include_final_newline: true))
+            corrector.remove(range_by_whole_lines(heredoc_end, include_final_newline: true))
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/layout/empty_comment_spec.rb

@@ -9,8 +9,7 @@
       ^ Source code comment is empty.
     RUBY
 
-    expect_correction(<<~RUBY)
-    RUBY
+    expect_correction('')
   end
 
   it 'registers an offense and corrects using multiline empty comments' do
@@ -21,8 +20,7 @@
       ^ Source code comment is empty.
     RUBY
 
-    expect_correction(<<~RUBY)
-    RUBY
+    expect_correction('')
   end
 
   it 'registers an offense and corrects using an empty comment next to code' do
@@ -83,8 +81,7 @@ def hello
         ^ Source code comment is empty.
       RUBY
 
-      expect_correction(<<~RUBY)
-      RUBY
+      expect_correction('')
     end
 
     it 'registers an offense and corrects using border comment' do
@@ -93,8 +90,7 @@ def hello
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Source code comment is empty.
       RUBY
 
-      expect_correction(<<~RUBY)
-      RUBY
+      expect_correction('')
     end
   end
 

spec/rubocop/cop/style/empty_heredoc_spec.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Style::EmptyHeredoc, :config do
+  it 'registers an offense when using empty `<<~EOS` heredoc' do
+    expect_offense(<<~RUBY)
+      <<~EOS
+      ^^^^^^ Use an empty string literal instead of heredoc.
+      EOS
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ''
+    RUBY
+  end
+
+  it 'registers an offense when using empty `<<-EOS` heredoc' do
+    expect_offense(<<~RUBY)
+      <<-EOS
+      ^^^^^^ Use an empty string literal instead of heredoc.
+      EOS
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ''
+    RUBY
+  end
+
+  it 'registers an offense when using empty `<<EOS` heredoc' do
+    expect_offense(<<~RUBY)
+      <<EOS
+      ^^^^^ Use an empty string literal instead of heredoc.
+      EOS
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ''
+    RUBY
+  end
+
+  it 'registers an offense when using empty heredoc single argument' do
+    expect_offense(<<~RUBY)
+      do_something(<<~EOS)
+                   ^^^^^^ Use an empty string literal instead of heredoc.
+      EOS
+    RUBY
+
+    expect_correction(<<~RUBY)
+      do_something('')
+    RUBY
+  end
+
+  it 'registers an offense when using empty heredoc argument with other argument' do
+    expect_offense(<<~RUBY)
+      do_something(<<~EOS, arg)
+                   ^^^^^^ Use an empty string literal instead of heredoc.
+      EOS
+    RUBY
+
+    expect_correction(<<~RUBY)
+      do_something('', arg)
+    RUBY
+  end
+
+  it 'does not register an offense when using not empty heredoc' do
+    expect_no_offenses(<<~RUBY)
+      <<~EOS
+        Hello.
+      EOS
+    RUBY
+  end
+end