fix(case): use Condition.parse_expression for empty/blank handling

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: charlespwd

lib/liquid/tags/case.rb

@@ -99,7 +99,7 @@ def parse_when(markup, body)
       parser = @parse_context.new_parser(markup)
 
       loop do
-        expr = parser.expression
+        expr = Condition.parse_expression(parser)
         block = Condition.new(@left, '==', expr)
         block.attach(body)
         @blocks << block