Use default MySQL credentials if username/pass not provided

Currently, if you don't specify the mysql_user and/or mysql_passwd parameters, the newrelic_plugins::mysql class will set them to empty strings in the mysql.instance.json file. Instead, it should just leave them out of the config and let the plugin use the defaults (see [mysql_user.sql](https://github.com/newrelic-platform/newrelic_mysql_java_plugin/blob/master/scripts/mysql_user.sql)). This simplifies things for the user and means you don't have to pass sensitive information to newrelic_plugins::mysql.