[CHEF-3270] Ensure path is not nil

Otherwise when running in empty env (env -i /usr/bin/chef-client), we're ending up with something looking like:
[Fri, 06 Jul 2012 16:51:04 +0200] INFO: *** Chef 0.10.10 ***
[Fri, 06 Jul 2012 16:51:04 +0200] FATAL: Stacktrace dumped to /var/cache/chef/chef-stacktrace.out
[Fri, 06 Jul 2012 16:51:04 +0200] FATAL: NoMethodError: private method `split' called for nil:NilClass


stacktrace.out:
<pre>
Generated at Fri Jul 06 17:01:18 +0200 2012
NoMethodError: private method `split' called for nil:NilClass
/usr/lib/ruby/vendor_ruby/chef/mixin/path_sanity.rb:26:in `enforce_path_sanity'
/usr/lib/ruby/vendor_ruby/chef/client.rb:151:in `run'
/usr/lib/ruby/vendor_ruby/chef/application/client.rb:254:in `run_application'
/usr/lib/ruby/vendor_ruby/chef/application/client.rb:241:in `loop'
/usr/lib/ruby/vendor_ruby/chef/application/client.rb:241:in `run_application'
/usr/lib/ruby/vendor_ruby/chef/application.rb:70:in `run'
</pre>