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