Crash on first install when using nginx::source

When installing nginx from source using `nginx::source`, recipe crashes on first install because `node.automatic_attrs['nginx']` is `nil`.

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# Trace

```
[2012-06-16T13:27:51-04:00] ERROR: Running exception handlers
[2012-06-16T13:27:51-04:00] ERROR: Exception handlers complete
[2012-06-16T13:27:51-04:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
[2012-06-16T13:27:51-04:00] FATAL: NoMethodError: bash[compile_nginx_source] (nginx::source line 79) had an error: NoMethodError: undefined method `[]' for nil:NilClass
```