(CAT-1724) - Adjust stringify_nil_attrs to include Enum

When comparing a resource from manifest that is an empty string
to the value set in the system, we have a mismatch between
empty_string and nil.

Similarly what we done for nil strings, we can extend to nil Enums
returned by DSC,

lib/puppet/provider/dsc_base_provider/dsc_base_provider.rb

@@ -666,15 +666,16 @@ def mandatory_set_attributes(context)
   end
 
   # Parses the DSC resource type definition to retrieve the names of any attributes which are specifed as required strings
+  #
   # This is used to ensure that any nil values are converted to empty strings to match puppets expecetd value
   # @param context [Object] the Puppet runtime context to operate in and send feedback to
   # @param data [Hash] the hash of properties returned from the DSC resource
   # @return [Hash] returns a data hash with any nil values converted to empty strings
   def stringify_nil_attributes(context, data)
     nil_strings = data.select { |_name, value| value.nil? }.keys
-    string_attrs = context.type.attributes.select { |_name, properties| properties[:type] == 'String' }.keys
-    string_attrs.each do |attribute|
-      data[attribute] = '' if nil_strings.include?(attribute)
+    attributes = context.type.attributes.select { |_name, properties| properties[:type].match?(/^Optional\[(String|Enum)|^(String|Enum)/) }
+    attributes.each do |name, _properties|
+      data[name] = '' if nil_strings.include?(name)
     end
     data
   end