Guard for nil data points

[atoulme]

Description:
Add a guard for invalid metrics with no data points. The function MetricAndDataPointCount currently panics if the gauge, histogram or sum is nil.

Testing:
I added a unit test and reproduced the issue, making sure the issue is fixed with the test passing.

[codecov]

Codecov Report

Merging #2055 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2055      +/-   ##
==========================================
+ Coverage   92.05%   92.07%   +0.01%     
==========================================
  Files         278      278              
  Lines       16811    16823      +12     
==========================================
+ Hits        15476    15490      +14     
+ Misses        918      917       -1     
+ Partials      417      416       -1     

Impacted Files	Coverage Δ
consumer/pdata/metric.go	99.31% <100.00%> (+0.06%)	⬆️
translator/internaldata/resource_to_oc.go	89.04% <0.00%> (-2.74%)	⬇️
receiver/otlpreceiver/otlp.go	92.30% <0.00%> (+2.56%)	⬆️
service/telemetry.go	85.00% <0.00%> (+3.33%)	⬆️

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