Apply suggestions from code review

Co-authored-by: NAITOH Jun <naitoh@gmail.com>

Author: tompng

lib/rexml/doctype.rb

@@ -123,7 +123,7 @@ def attribute_of element, attribute
       _attlist_mappings[element]&.[](attribute)
     end
 
-    def _attlist_mappings
+    def _attlist_mappings # :nodoc:
       mapping = {}
       grep(AttlistDecl).each do |child|
         raw_attributes = mapping[child.element_name] ||= {}

lib/rexml/element.rb

@@ -2523,7 +2523,8 @@ def _calculate_namespaces(attlist_mappings = nil) # :nodoc:
     # the element's attribute takes precedence.
     def each_effective_attribute(attlist_mappings = nil)
       return to_enum(__method__, attlist_mappings) unless block_given?
-      attributes = attlist_attributes(attlist_mappings).merge(self)
+      attlist = attlist_attributes(attlist_mappings)
+      attributes = attlist.empty? ? self : attlist.merge(self)
       attributes.each_value do |attribute|
         yield attribute
       end