Do not return pointers from windowing functions

Commit c2f322e introduced large regressions
since due to two changes:
* Changed return values of windowing functions from float64 to *float64, causing
excessive allocations.
* Added an expensive check on the hot path for a condition that could be calculated
once for the entire query.

This commit reverts that changes and adds a better way to handle cases where both
float and histogram values are missing, but there is no error in the evaluation.

Signed-off-by: Filip Petkovski <[email protected]>

Author: fpetkovski

execution/scan/subquery.go

@@ -201,8 +201,8 @@ func (o *subqueryOperator) Next(ctx context.Context) ([]model.StepVector, error)
 			if ok {
 				if h != nil {
 					sv.AppendHistogram(o.pool, uint64(sampleId), h)
-				} else if f != nil {
-					sv.AppendSample(o.pool, uint64(sampleId), *f)
+				} else {
+					sv.AppendSample(o.pool, uint64(sampleId), f)
 				}
 			}
 			o.IncrementSamplesAtTimestamp(rangeSamples.Len(), sv.T)