Avoid warning if no rewrite rules exist

Currently running "wp rewrite dump" against a site that has never had permalinks set throws a warning since the return value from the get_option call is false, but is assumed to be an array, and is not checked before used in a foreach()

The attached PR amends the code to avoid the warning. It also goes a bit further and produces some default output "No rules" in the case where there are no rules configured, both in the JSON, and non-JSON case. 

If you'd prefer it to output nothing happy to rework the PR...