Support fill(previous) where first value is missing and outside of time bounds.

[adamperlin]

From ifql created by nathanielc : influxdata/ifql#210

influxdata/influxdb#5943

[aanthony1243]

related to #272

[SGStino]

first value missing also applies to non-windowed queries:

previous = from(bucket: "Data")
  |> range(stop: v.timeRangeStart, start: 0) 
  |> filter(fn: (r) => r["_measurement"] == "Tags.PLC_252.TANK1.STAPNR")
  |> filter(fn: (r) => r["_field"] == "value")  
  |> last()
  
current = from(bucket: "Data")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "Tags.PLC_252.TANK1.STAPNR")
  |> filter(fn: (r) => r["_field"] == "value")  

union(tables: [previous, current])  

(aside of an issue that this results in two tables in the final result set), a shortcut would be nice there too.
not sure if fill(use_previous:true) should fix this though, or something else?

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