Make HybridTag#blank? abstract via NotImplementedError

Subclasses must now explicitly implement blank? instead of inheriting
a default true. TestHybridTag updated to provide its own implementation.

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

Author: charlespwd

lib/liquid/hybrid_tag.rb

@@ -22,7 +22,7 @@ def nodelist
     end
 
     def blank?
-      true
+      raise NotImplementedError, "#{self.class} must implement blank?"
     end
 
     def render_to_output_buffer(context, output)

test/unit/hybrid_tag_unit_test.rb

@@ -6,6 +6,10 @@ class HybridTagUnitTest < Minitest::Test
   include Liquid
 
   class TestHybridTag < Liquid::HybridTag
+    def blank?
+      true
+    end
+
     private
 
     def render_self_closing_to_output_buffer(_context, output)