[BEAM-13867] Drop NaNs returned by nlargest in flight_delays example pipeline

[TheNeuralBit]

pandas 1.4.0 slightly changed the behavior of nlargest for pandas-dev/pandas#28984 - now it can return NaNs in some specific cases. This PR modifies our flight_delays example to explicitly drop those NaNs.

ValidatesRunner compliance status (on master branch)

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

Examples testing status on various runners

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

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

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.

[TheNeuralBit]

Run Python 3.8 PostCommit

[codecov]

Codecov Report

Merging #16801 (b315955) into master (16355ca) will increase coverage by 37.23%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master   #16801       +/-   ##
===========================================
+ Coverage   46.38%   83.62%   +37.23%     
===========================================
  Files         202      452      +250     
  Lines       19828    62252    +42424     
===========================================
+ Hits         9198    52059    +42861     
- Misses       9643    10193      +550     
+ Partials      987        0      -987     

Impacted Files	Coverage Δ
...on/apache_beam/examples/dataframe/flight_delays.py	27.90% <0.00%> (ø)
sdks/go/pkg/beam/io/databaseio/database.go
sdks/go/pkg/beam/io/synthetic/step.go
sdks/go/pkg/beam/io/filesystem/filesystem.go
sdks/go/pkg/beam/core/runtime/graphx/serialize.go
sdks/go/pkg/beam/transforms/stats/quantiles.go
sdks/go/pkg/beam/core/runtime/exec/plan.go
.../pkg/beam/runners/universal/extworker/extworker.go
sdks/go/pkg/beam/core/util/protox/base64.go
sdks/go/pkg/beam/core/runtime/harness/datamgr.go
... and 645 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 16355ca...b315955. Read the comment docs.

[tvalentyn]

Run Python Examples_Direct