FIX: Prevent app crash when it `take`s nil objects

When a command attributes expires, it's ID still remains
in the redis global IDs set. This can cause inconsistencies in commands
like `#take`.

For the moment, we will skip the nil results from search commands, but
the inconsistency should be fixed outside the app
(maybe in a background process).
Merge pull request #6 from wuakitv/fix-crash-when-taking-nil-objects

FIX: Prevent app crash when it `take`s nil objects