Fix Describable returning an empty list

[fstab]

As described in #780, if a Collector implements Describable, but the describe() method returns an empty list, the metrics from this Collector will never be collected.

This PR should fix this.

[yaronel]

Maybe I am missing something but it doesn't seem to fix the issue.
The problem is the List<String> of names is empty, so this inner loop is not going to run.
One way to make it work, though I don't know if this is the behavior you want, is something like this:

          for (Map.Entry<Collector, List<String>> entry : collectorsToNames.entrySet()) {
            if (entry.getValue().isEmpty()) {
              collectors.add(entry.getKey());
            } else {
              for (String name : entry.getValue()) {
                if (sampleNameFilter.test(name)) {
                  collectors.add(entry.getKey());
                  break;
                }
              }
            }
          }

[fstab]

Thanks a lot @yaronel. I fixed it and added a test to make sure it works.