[Fix rubocop#11998] Fix an error when inspecting blank heredoc delimiter

Fixes rubocop#11998.

This PR fixes an error when inspecting blank heredoc delimiter.

changelog/fix_an_error_when_inspecting_blank_heredoc_delimiter.md

@@ -0,0 +1 @@
+* [#11998](https://github.com/rubocop/rubocop/issues/11998): Fix an error when inspecting blank heredoc delimiter. ([@koic][])

lib/rubocop/cop/mixin/heredoc.rb

@@ -26,11 +26,15 @@ def indent_level(str)
       end
 
       def delimiter_string(node)
-        node.source.match(OPENING_DELIMITER).captures[1]
+        return '' unless (match = node.source.match(OPENING_DELIMITER))
+
+        match.captures[1]
       end
 
       def heredoc_type(node)
-        node.source.match(OPENING_DELIMITER).captures[0]
+        return '' unless (match = node.source.match(OPENING_DELIMITER))
+
+        match.captures[0]
       end
     end
   end

spec/rubocop/cop/naming/heredoc_delimiter_case_spec.rb

@@ -173,6 +173,14 @@
           RUBY
         end
       end
+
+      context 'when using blank heredoc delimiters' do
+        it 'does not register an offense' do
+          expect_no_offenses(<<~RUBY)
+            <<''
+          RUBY
+        end
+      end
     end
 
     context 'with a squiggly heredoc' do

spec/rubocop/cop/style/redundant_heredoc_delimiter_quotes_spec.rb

@@ -161,4 +161,10 @@
       EOS
     RUBY
   end
+
+  it 'does not register an offense when using the blank heredoc delimiter' do
+    expect_no_offenses(<<~RUBY)
+      <<''
+    RUBY
+  end
 end