Fix errors caught by Dialyzer

[Stratus3D]

It doesn't look like Dialyzer is getting run for this project.

Below are the fixes for the errors reported by Dialyzer:

lib/broadway_kafka/brod_client.ex:83:guard_fail
The guard clause:

when _ :: Keyword.t() === nil

can never succeed.

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lib/broadway_kafka/brod_client.ex:114:guard_fail
The guard clause:

when _ :: pid() === nil

can never succeed.

________________________________________________________________________________
lib/broadway_kafka/kafka_client.ex:54:unknown_type
Unknown type: :brod.group_coordinator/0.
________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2

[josevalim]

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[Stratus3D]

Thanks @josevalim !