Puppet users - Mar 2014 - how do I check to see whether a hash value is defined if a parent of the hash value might not even exist?

I've got a big hash of info that looks like this:

  $web_conf = {
    my_client => {
      dev => { 
        ws => {
          server_name => 'ws.foo.bar',
          server_aliases => ['ws.dev.bar.foo.com', 
'ws.dev.another.foo.com'], 
        },
        csr => { 
          server_aliases => ['csr.dev.bar.foo.com', 
'csr.dev.another.foo.com'], 
        },
        mw => { 
          server_aliases => ['mw.dev.bar.foo.com', 
'mw.dev.another.foo.com'], 
        },
      },
    }

I'm trying to do something like this in my manifest:

  # set up our servername
  if $web_conf[$::client][$::env][$name]['server_name'] {
    $server_name = $web_conf[$::client][$::env][$name]['server_name']
  } else {
    $server_name = "${name}.${client_code}.${::env}.${domain}"
  }

The thought was that I'd have servers that follow a certain naming 
convention and ones that don't. The ones that have 'special' names
go in
the web_conf hash. If the manifest 'sees' a server name in the hash, it 
should use that. If not, then it just does the 'else' bit.

The trouble is that it totally fails if even the top or intermediate levels 
of the hash don't exist. I've tried using "if 
$web_conf[$::client][$::env][$name]['server_name'] != undef", tried
"if
undef($web_conf[$::client][$::env][$name]['server_name'])" etc. 

what's the correct way to do this?
--steve  

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