Make PromDash deal with empty metric names.

The Prometheus server is being changed to drop metric names after
transformations like sum(), avg(), etc., since the result of these
transformations is not the original metric anymore. Thus, the metric
name of timeseries may be empty now. PromDash needs to deal with this.
Ah no, thanks. That was a remnant of debugging https://github.com/prometheus/promdash/issues/213, which I discovered during this change. Removed.