Fix exception when worker hostname is not defined

[oriolpiera]

Previous

When a Worker has not defined the attribute hostname, it raise an exception and the list of workers is not showed

After

The list of Workers appears and the field hostname in workers without this attributed, is showed blank

[pranavgupta1234]

I think it would be better to show this case also as "Unresolved" (although its not exactly similar to case when IP is not resolved from given hostname) but showing two behaviour like leaving blank and showing unresolved might be confusing.

[oriolpiera]

That works for me