Correctly test for plugin if it is nil

Author: dveeden

lib/puppet/provider/mysql_user/mysql.rb

@@ -50,7 +50,7 @@ def create
 
     # Use CREATE USER to be compatible with NO_AUTO_CREATE_USER sql_mode
     # This is also required if you want to specify a authentication plugin
-    if defined? plugin
+    if !plugin.nil?
       mysql([defaults_file, '-e', "CREATE USER '#{merged_name}' IDENTIFIED WITH '#{plugin}'"].compact)
       @property_hash[:ensure] = :present
       @property_hash[:plugin] = plugin