Don't crash on rabbitmq_plugins_dirs fact if rabbitmqctl is not present
Don't crash on rabbitmq_plugins_dirs fact if rabbitmqctl is not present

Before trying to access the contents of a regexp match, see if the
match was successful. On machines without rabbitmqctl this match will
return `nil`, so we can't index it with `[1]`. When that's the case,
the whole rabbitmq_plugins_dirs fact value will now be `nil`, instead
of throwing an exception.

Fixes: #783

puppetlabs/puppetlabs-rabbitmq,Slm0n87/puppet-rabbitmq,voxpupuli/puppet-rabbitmq,Slm0n87/puppet-rabbitmq,Slm0n87/puppet-rabbitmq,puppetlabs/puppetlabs-rabbitmq,voxpupuli/puppet-rabbitmq