Fix nginx::params deprecation notice

When calling `class { 'nginx': }` from outside the nginx module and because `nginx` inherits from `nginx::params` it is triggering [this statement](https://github.com/jfryman/puppet-nginx/blob/master/manifests/params.pp#L18-L20) incorrectly.

`$caller_module_name` is undefined when `class { 'nginx': }` is called in the global scope (i.e. within your `site.pp`) so this check for that first to avoid the false matches.