Have 'Hide series with only zeros' ignore nulls (#9179)

* Prometheus: Fix actual step computation logic when a min_step is specified and the range is longer than min_step * 11000.

* Have the 'Hide series with only zeros' option also apply to series with some null values.

public/app/core/time_series2.ts

@@ -191,11 +191,10 @@ export default class TimeSeries {
           this.stats.logmin = currentValue;
         }
 
-      }
-
         if (currentValue !== 0) {
           this.allIsZero = false;
         }
+      }
 
       result.push([currentTime, currentValue]);
     }