[BEAM-9547] Add tests for groupby(dropna)

[TheNeuralBit]

Post-Commit Tests Status (on master branch)

Lang	SDK	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go		---			---		---
Java
Python		---			---		---
XLang		---			---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---			---	---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[codecov]

Codecov Report

Merging #14553 (293ae30) into master (985e2f0) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #14553   +/-   ##
=======================================
  Coverage   83.61%   83.62%           
=======================================
  Files         445      445           
  Lines       59025    59025           
=======================================
+ Hits        49354    49358    +4     
+ Misses       9671     9667    -4     

Impacted Files	Coverage Δ
.../sdks/python/apache_beam/io/external/gcp/pubsub.py
...tes/tox/py38/build/srcs/sdks/python/test_config.py
...ython/apache_beam/io/external/generate_sequence.py
...les/complete/juliaset/juliaset/juliaset_test_it.py
...ernal/clients/cloudbuild/cloudbuild_v1_messages.py
...ow/dataflow_exercise_streaming_metrics_pipeline.py
...thon/apache_beam/runners/worker/operation_specs.py
.../python/apache_beam/io/gcp/datastore/v1new/util.py
...he_beam/transforms/combinefn_lifecycle_pipeline.py
...38/build/srcs/sdks/python/apache_beam/io/avroio.py
... and 880 more

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[TheNeuralBit]

Failures look like flake BEAM-12163