Fix an error for Style/GuardClause

This PR fixes the following error for `Style/GuardClause`
when using heredoc as an argument of raise in `then` branch
and it does not have `else` branch:

```ruby
def func
  if condition
    raise <<~MESSAGE
      oops
    MESSAGE
  end
end
```

```console
% bundle exec rubocop --only Style/GuardClause -d
(snip)

An error occurred while Style/GuardClause cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/guard_clause/example.rb:1:0.
undefined method `source_range' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/guard_clause.rb:209:in `autocorrect_heredoc_argument'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/guard_clause.rb:191:in `autocorrect'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/guard_clause.rb:179:in `block in register_offense'
```

changelog/fix_error_for_style_guard_clause.md

@@ -0,0 +1 @@
+* [#11344](https://github.com/rubocop/rubocop/pull/11344): Fix an error for `Style/GuardClause` when using heredoc as an argument of raise in `then` branch and it does not have `else` branch. ([@koic][])

lib/rubocop/cop/style/guard_clause.rb

@@ -196,7 +196,7 @@ def autocorrect(corrector, node, condition, replacement, guard)
             return unless node.else?
 
             corrector.remove(node.loc.else)
-            corrector.remove(branch_to_remove(node, guard))
+            corrector.remove(range_of_branch_to_remove(node, guard))
           end
         end
         # rubocop:enable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity
@@ -206,20 +206,24 @@ def heredoc?(argument)
         end
 
         def autocorrect_heredoc_argument(corrector, node, heredoc_branch, leave_branch, guard)
+          return unless node.else?
+
           remove_whole_lines(corrector, leave_branch.source_range)
           remove_whole_lines(corrector, node.loc.else)
           remove_whole_lines(corrector, node.loc.end)
-          remove_whole_lines(corrector, branch_to_remove(node, guard).source_range)
+          remove_whole_lines(corrector, range_of_branch_to_remove(node, guard))
           corrector.insert_after(
             heredoc_branch.last_argument.loc.heredoc_end, "\n#{leave_branch.source}"
           )
         end
 
-        def branch_to_remove(node, guard)
-          case guard
-          when :if then node.if_branch
-          when :else then node.else_branch
-          end
+        def range_of_branch_to_remove(node, guard)
+          branch = case guard
+                   when :if then node.if_branch
+                   when :else then node.else_branch
+                   end
+
+          branch.source_range
         end
 
         def guard_clause_source(guard_clause)

spec/rubocop/cop/style/guard_clause_spec.rb

@@ -297,6 +297,31 @@ def func
     RUBY
   end
 
+  it 'registers an offense when using heredoc as an argument of raise in `then` branch and it does not have `else` branch' do
+    expect_offense(<<~'RUBY')
+      def func
+        if condition
+        ^^ Use a guard clause (`return unless condition`) instead of wrapping the code inside a conditional expression.
+          raise <<~MESSAGE
+            oops
+          MESSAGE
+        end
+      end
+    RUBY
+
+    # NOTE: Let `Layout/HeredocIndentation`, `Layout/ClosingHeredocIndentation`, and
+    #       `Layout/IndentationConsistency` cops autocorrect inconsistent indentations.
+    expect_correction(<<~RUBY)
+      def func
+        return unless condition
+          raise <<~MESSAGE
+            oops
+          MESSAGE
+        end
+      end
+    RUBY
+  end
+
   it 'registers an offense when using xstr heredoc as an argument of raise in `else` branch' do
     expect_offense(<<~RUBY)
       def func