Derive block_form? from @Body presence instead of tracking separately

@Body is only set in reparent_as_block, so it already encodes whether
the tag is in block form. Remove the redundant @block_form flag.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: charlespwd

lib/liquid/hybrid_tag.rb

@@ -3,18 +3,16 @@
 module Liquid
   class HybridTag < Block
     def reparent_as_block(children, parse_context)
-      @block_form = true
       @body = new_body
       @body.nodelist.concat(children)
       @body.freeze
     end
 
     def parse(_tokens)
-      @block_form = false
     end
 
     def block_form?
-      @block_form
+      !!@body
     end
 
     def nodelist
@@ -26,7 +24,7 @@ def blank?
     end
 
     def render_to_output_buffer(context, output)
-      if @block_form
+      if block_form?
         render_block_form_to_output_buffer(context, output)
       else
         render_self_closing_to_output_buffer(context, output)