Gauge visual to have custom "show as blank" field

[David_Plajner]

Just like on the new cards, gauges should have a custom field to define what should be shown if the source happens to be null.

Currently it shows (blank) and coalesce(value, 0) may not always be reasonable to use therefore the only reliable fallback is fx and set to bg-color when blank. But this is ugly. I'd hope to have a small update added where (blank) is -- by default on the gauge but also customisable jsut like on the new cards.  

Thanks!