[SPARK-12611][SQL][PYSPARK][TESTS] Fix test_infer_schema_to_local

Previously (when the PR was first created) not specifying b= explicitly was fine (and treated as default null) - instead be explicit about b being None in the test.

Author: Holden Karau <holden@us.ibm.com>

Closes #10564 from holdenk/SPARK-12611-fix-test-infer-schema-local.