Skip creation of redundant nil resource in translation from OC if there are no combined metrics

[nilebox]

Description: Getting rid of redundant nil ResourceObject when all metrics have resource specified.

After removal of pdatautil package in #1739, I've migrated our custom components from pdatautil.MetricsFromMetricsData to internaldata.OCToMetrics and found that some tests are failing due to converted metrics having an extra "empty" ResourceMetrics object in the slice.

Testing: Added unit test reproducing the issue

/cc @bogdandrutu

[codecov]

Codecov Report

Merging #1803 into master will decrease coverage by 0.89%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1803      +/-   ##
==========================================
- Coverage   91.66%   90.76%   -0.90%     
==========================================
  Files         262      262              
  Lines       18603    15914    -2689     
==========================================
- Hits        17052    14445    -2607     
+ Misses       1118     1039      -79     
+ Partials      433      430       -3     

Impacted Files	Coverage Δ
translator/internaldata/oc_to_metrics.go	92.38% <100.00%> (-0.61%)	⬇️
service/internal/templates.go	30.43% <0.00%> (-21.09%)	⬇️
processor/cloningfanoutconnector.go	57.57% <0.00%> (-9.10%)	⬇️
cmd/mdatagen/metricdata.go	71.42% <0.00%> (-8.29%)	⬇️
exporter/kafkaexporter/otlp_marshaller.go	71.42% <0.00%> (-6.35%)	⬇️
...rnal/scraper/processesscraper/processes_scraper.go	80.00% <0.00%> (-5.72%)	⬇️
extension/pprofextension/pprofextension.go	57.14% <0.00%> (-5.36%)	⬇️
service/builder/extensions_builder.go	80.76% <0.00%> (-4.53%)	⬇️
...eceiver/internal/scraper/processscraper/factory.go	55.55% <0.00%> (-4.45%)	⬇️
receiver/otlpreceiver/marshal_jsonpb.go	12.61% <0.00%> (-4.45%)	⬇️
... and 245 more

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