Add default rows method to RowGroup base class

Prevents NoMethodError when RowGroup is instantiated
directly, since render? calls rows.any?.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: myabc

app/components/primer/open_project/table/row_group.rb

@@ -15,6 +15,10 @@ def initialize(**system_arguments)
           @system_arguments[:role] = :rowgroup
         end
 
+        def rows
+          []
+        end
+
         def render?
           rows.any? || content?
         end