mgr/dashboard: Add text to empty life expectancy column
The Life Expectancy column is empty if the disk prediction mgr module is disabled. It might also be empty when there's no prediction but the prediction is enabled. But it could also be that the status "unknown" is used when disk prediction is enabled but insufficient data has been gathered so far.
Those possible values should be aligend with a new "empty value" for the "Life Expectancy" column. If "unknown" is a status returned by the diskprediction module, we should probably use something different to make it clear that "unknown" is a state that lacks data to be able to create a prediction but "n/a" (or something else, may be in the direction of "disabled") is used to display that this feature is not enabled.
Maybe it makes even sense to hide than column, but that likely wouldn't align with how the dashboard handles unconfigured features. The dashboard mostly tells the user that a feature is disabled and how to enable it.