Fix NPE from dell_cache existing, but the key not so much

If we request a value that the dell_cache doesn't have, it really shouldn't throw an NPE, hosing the entire facter (and thus puppet) run.  This actually happened on the prd3-vm-01 machine.
Merge pull request #1 from peculater/patch-1

Fix NPE from dell_cache existing, but the key not so much