don't crash if source block with custom subs is empty and source high…

…lighter is enabled

CHANGELOG.adoc

@@ -7,7 +7,9 @@ For a detailed view of what has changed, refer to the {url-repo}/commits/main[co
 
 == Unreleased
 
-_No changes since previous release._
+Bug Fixes::
+
+* don't crash if source block with custom subs is empty and source highlighter is enabled
 
 == 2.3.6 (2023-04-09) - @mojavelinux
 

lib/asciidoctor/pdf/converter.rb

@@ -1079,8 +1079,9 @@ def convert_code node
                   callouts_enabled = false
                 end
                 subs[highlight_idx] = :specialcharacters
+                source_string = node.content || ''
                 # NOTE: indentation guards will be added by the source highlighter logic
-                source_string = expand_tabs unescape_xml (sanitize node.content, compact: false)
+                source_string = expand_tabs unescape_xml (sanitize source_string, compact: false) unless source_string.empty?
                 node.lines.replace saved_lines if saved_lines
               end
             else

spec/source_spec.rb

@@ -1225,6 +1225,20 @@ class Type; end
       (expect attr_name_text[:font_color]).to eql '4F9FCF'
       (expect (pdf.find_text '①')[0]).not_to be_nil
     end
+
+    it 'should not crash if source highlighter is enabled and subs is set on empty block' do
+      (expect do
+        input = <<~'END'
+        [source,ruby,subs=attributes+]
+        ----
+        ----
+        END
+        lines = (to_pdf input, attribute_overrides: { 'source-highlighter' => 'coderay' }, analyze: :line).lines
+        text = (to_pdf input, attribute_overrides: { 'source-highlighter' => 'coderay' }, analyze: true).text
+        (expect (lines[0][:from][:y] - lines[1][:from][:y]).abs).to be < 5
+        (expect text).to be_empty
+      end).not_to raise_exception
+    end
   end
 
   context 'Pygments' do