UDF jar tests should fail if system properties are unset.

In BEAM-11691, I skipped tests for UDF jar tests if system properties beam.sql.udf.test.jar_path and beam.sql.udf.test.empty_jar_path are unset. However, this puts us at risk of silently ignoring these tests. The proper solution is to fail if the properties are unset, which will require setting the properties for the test variants that I missed the first time around.