Preserve literal semantics in strict2 case/when

Previously, strict2 case/when used `safe_parse_expression`
to parse when expressions causing `blank`/`empty` to be
treated as string literals (Expression::LITERALS maps 'empty' => ''),
rather than method literals

This caused unexpected behavior:

```
{%- case empty_obj -%}
{%- when empty -%}
  previously: doesn't render (empty_obj == '' is false)
  now: renders (empty_obj.empty? is true)
{%- endcase -%}
```

This commit instead calls `Condition.parse_expression`
with `safe: true`, which will correctly handle `blank`
and `empty`

test/unit/tags/case_tag_unit_test.rb

@@ -82,6 +82,26 @@ class CaseTagUnitTest < Minitest::Test
     end
   end
 
+  def test_case_when_empty
+    template = <<~LIQUID
+      {%- case x -%}
+        {%- when 2 or empty -%}
+          2 or empty
+        {%- else -%}
+          not 2 or empty
+      {%- endcase -%}
+    LIQUID
+
+    with_error_modes(:lax, :strict, :strict2) do
+      assert_template_result("2 or empty", template, { 'x' => 2 })
+      assert_template_result("2 or empty", template, { 'x' => {} })
+      assert_template_result("2 or empty", template, { 'x' => [] })
+      assert_template_result("not 2 or empty", template, { 'x' => { 'a' => 'b' } })
+      assert_template_result("not 2 or empty", template, { 'x' => ['a'] })
+      assert_template_result("not 2 or empty", template, { 'x' => 4 })
+    end
+  end
+
   def test_case_with_invalid_expression
     template = <<~LIQUID
       {%- case foo=>bar -%}