Fix Doc tag blank? method to check body content

Previously the blank? method always returned true. Now it properly checks
if the body is empty, making the tag behavior consistent with other tags.

Also updated test to use whitespace control for cleaner assertions.

lib/liquid/tags/doc.rb

@@ -59,7 +59,7 @@ module Liquid
     end
 
     def blank?
-      true
+      @body.empty?
     end
 
     def nodelist

test/unit/tags/doc_tag_unit_test.rb

@@ -174,11 +174,11 @@ class DocTagUnitTest < Minitest::Test
 
   def test_doc_tag_delimiter_handling
     assert_template_result('', <<~LIQUID.chomp)
-      {% if true %}
-        {% doc %}
-          {% docEXTRA %}wut{% enddocEXTRA %}xyz
-        {% enddoc %}
-      {% endif %}
+      {%- if true -%}
+        {%- doc -%}
+          {%- docEXTRA -%}wut{% enddocEXTRA -%}xyz
+        {%- enddoc -%}
+      {%- endif -%}
     LIQUID
 
     assert_template_result('', "{% doc %}123{% enddoc xyz %}")