Do not export aggregations without any data points

[MrAlias]

Fix #3394

Currently, the SDK exports histograms, last-values, and sums that do not contain any data points. This updates the aggregation implementations to return nil instead of values if that is the case. From there this will prevent empty data from being exported:

opentelemetry-go/sdk/metric/pipeline.go

Lines 134 to 135 in 49b62ae

	data := inst.aggregator.Aggregation()
	if data != nil {

[codecov]

Codecov Report

Merging #3436 (c1d41bd) into main (acd8777) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3436   +/-   ##
=====================================
  Coverage   77.7%   77.7%           
=====================================
  Files        164     164           
  Lines      11520   11521    +1     
=====================================
+ Hits        8952    8957    +5     
+ Misses      2370    2368    -2     
+ Partials     198     196    -2     

Impacted Files	Coverage Δ
sdk/metric/internal/histogram.go	100.0% <100.0%> (+2.1%)	⬆️
sdk/metric/internal/lastvalue.go	100.0% <100.0%> (ø)
sdk/metric/internal/sum.go	100.0% <100.0%> (+2.4%)	⬆️
exporters/jaeger/jaeger.go	90.3% <0.0%> (-0.9%)	⬇️