Prevent 'Use of uninitialized value' warnings

[willvincent]

When processes running within docker containers are running as users without an entry in the host machine's /etc/passwd file, many warnings will appear in the log. While they're just warnings and could be ignored, they do produce a great deal of excess noise that would be better to prevent.

One solution would be to create user entries in the passwd file, but that really shouldn't be necessary. Instead we can just cast the user value to an empty string to prevent the warnings entirely.

This resolves #24

[wschaub]

Might be useful to just display the raw UID number instead of an empty string if it's not found in the password database. it's what ps(1) would do and what I would expect.

[willvincent]

Whatever the case.. linode doesn't seem terribly interested in continuing to support this codebase, so I guess it's on those of us affected to implement the solution we prefer, but probably not expect that this PR will ever be merged.