[BEAM-10529] update KafkaIO Xlang integration test to publish and receive null keys

[johnjcasey]

Update the kafkaIO xlang integration test to publish and receive a null key instead of an empty byte string. This will validate the changes to KafkaIO Xlang that enables nullable keys and values

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Codecov Report

Merging #17319 (0b1199e) into master (70ff734) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17319      +/-   ##
==========================================
+ Coverage   74.14%   74.18%   +0.04%     
==========================================
  Files         676      684       +8     
  Lines       89008    89659     +651     
==========================================
+ Hits        65992    66516     +524     
- Misses      21869    21996     +127     
  Partials     1147     1147              

Flag	Coverage Δ
python	83.64% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...pache_beam/runners/interactive/interactive_beam.py	77.81% <0.00%> (-3.28%)	⬇️
sdks/python/apache_beam/utils/interactive_utils.py	90.24% <0.00%> (-2.44%)	⬇️
sdks/python/apache_beam/coders/typecoders.py	92.40% <0.00%> (-2.27%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	90.84% <0.00%> (-1.41%)	⬇️
...ache_beam/runners/portability/expansion_service.py	91.83% <0.00%> (-1.35%)	⬇️
sdks/python/apache_beam/io/parquetio.py	95.16% <0.00%> (-0.93%)	⬇️
sdks/python/apache_beam/transforms/core.py	91.84% <0.00%> (-0.81%)	⬇️
sdks/python/apache_beam/transforms/combiners.py	93.03% <0.00%> (-0.39%)	⬇️
.../apache_beam/io/gcp/datastore/v1new/datastoreio.py	86.16% <0.00%> (-0.29%)	⬇️
...he_beam/runners/interactive/sql/beam_sql_magics.py	49.51% <0.00%> (-0.24%)	⬇️
... and 97 more

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[johnjcasey]

R: @chamikaramj

[johnjcasey]

R: @pabloem

[ahmedabu98]

LGTM

[johnjcasey]

Run PythonLint PreCommit

[aaltay]

Could this be merged?

[johnjcasey]

Yes, it can. @chamikaramj could you take another look?

[johnjcasey]

@chamikaramj @pabloem could one of you take a look at this?

[chamikaramj]

LGTM. Thanks.