transform.go: add drop_empty_series() function

This function can be used for dropping empty series before performing additional calculations.
For example, the following query drops `temperature` series with values bigger than 30,
while replacing gaps in the remaining series with 40.

   drop_empty_series(temperature <= 30) default 40

Updates VictoriaMetrics/VictoriaMetrics#5071

transform.go

@@ -29,6 +29,7 @@ var transformFuncs = map[string]bool{
 	"days_in_month":              true,
 	"deg":                        true,
 	"drop_common_labels":         true,
+	"drop_empty_series":          true,
 	"end":                        true,
 	"exp":                        true,
 	"floor":                      true,