Fix empty? semantics and string first/last for empty strings

- nil is NOT empty (but IS blank) - matches Shopify production
- String first/last returns '' for empty strings, not nil - matches ActiveSupport
- Add test for nil not being empty

lib/liquid/condition.rb

@@ -160,10 +160,9 @@ module Liquid
     end
 
     # Implement empty? semantics
+    # Note: nil is NOT empty (but IS blank). empty? checks if a collection has zero elements.
     def liquid_empty?(value)
       case value
-      when NilClass
-        true
       when String, Array, Hash
         value.empty?
       else

lib/liquid/standardfilters.rb

@@ -768,7 +768,8 @@ module Liquid
     # @liquid_syntax array | first
     # @liquid_return [untyped]
     def first(array)
-      return array[0] if array.is_a?(String)
+      # ActiveSupport returns "" for empty strings, not nil
+      return array[0] || "" if array.is_a?(String)
       array.first if array.respond_to?(:first)
     end
 
@@ -780,7 +781,8 @@ module Liquid
     # @liquid_syntax array | last
     # @liquid_return [untyped]
     def last(array)
-      return array[-1] if array.is_a?(String)
+      # ActiveSupport returns "" for empty strings, not nil
+      return array[-1] || "" if array.is_a?(String)
       array.last if array.respond_to?(:last)
     end
 

lib/liquid/variable_lookup.rb

@@ -71,8 +71,9 @@ module Liquid
           object = object.send(key).to_liquid
 
         # Handle string first/last like ActiveSupport does (returns first/last character)
+        # ActiveSupport returns "" for empty strings, not nil
         elsif lookup_command?(i) && object.is_a?(String) && (key == "first" || key == "last")
-          object = key == "first" ? object[0] : object[-1]
+          object = key == "first" ? (object[0] || "") : (object[-1] || "")
 
           # No key was present with the desired value and it wasn't one of the directly supported
           # keywords either. The only thing we got left is to return nil or

test/integration/standard_filter_test.rb

@@ -635,10 +635,12 @@ class StandardFiltersTest < Minitest::Test
     # This enables template patterns like:
     #   {{ product.title | first }}  => "S" (for "Snowboard")
     #   {{ customer.name | last }}   => "h" (for "Smith")
+    #
+    # Note: ActiveSupport returns "" for empty strings, not nil.
     assert_equal('f', @filters.first('foo'))
     assert_equal('o', @filters.last('foo'))
-    assert_nil(@filters.first(''))
+    assert_equal('', @filters.first(''))
-    assert_nil(@filters.last(''))
+    assert_equal('', @filters.last(''))
   end
 
   def test_first_last_on_unicode_strings

test/unit/condition_unit_test.rb

@@ -353,6 +353,16 @@ class ConditionUnitTest < Minitest::Test
     assert_evaluates_true(VariableLookup.new('empty_hash'), '==', empty_literal)
   end
 
+  def test_nil_is_not_empty
+    # nil is NOT empty - empty? checks if a collection has zero elements.
+    # nil is not a collection, so it cannot be empty.
+    # This differs from blank: nil IS blank, but nil is NOT empty.
+    @context['nil_value'] = nil
+    empty_literal = Condition.class_variable_get(:@@method_literals)['empty']
+
+    assert_evaluates_false(VariableLookup.new('nil_value'), '==', empty_literal)
+  end
+
   private
 
   def assert_evaluates_true(left, op, right)