Make sure @ciphers isn't an empty array

Make this module not break sshd with the default ciphers list

I just upgraded to the most recent version of this module and ran puppet, expecting no change.  What I got instead was a small change to /etc/ssh/sshd_config that kept sshd from starting :(

The problem is in templates/sshd_config.erb, lines 83-85:
```erb
<% if @ciphers -%>
Ciphers <%= @ciphers.join(",") %>
<% end -%>
```
manifests/server.pp line 18 is: 
```puppet
$ciphers=[],
```
Which exists but is empty :(

I just changed the check to make sure there's something in that array.

An equally-reasonable fix would be to default ```$ciphers``` to ```undef``` instead.  I'm sure there's some style guide entry somewhere that says, but I don't know.