fix test added in "python sdk: fix several bugs regarding avto <-> beam schema conversion"

[benkonz]

fix failing apache_beam/io/avroio_test.py::TestFastAvro::test_avro_schema_to_beam_schema_ with_nullable_atomic_fields test. It was failing when using the Flink runner in the :runners:flink:1.15:job-server:validatesCrossLanguageRunnerPythonUsingSql gradle task because the Flink runner isn't accessing the local file system when opening files.

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