Fix NPE in TestFlinkOrcReaderWriter

This is a regression at Apache ORC 1.7.0

• https://github.com/dongjoon-hyun/iceberg/pull/8/checks?check_run_id=3475456844

  org.apache.iceberg.flink.data.TestFlinkOrcReaderWriter > testMixedTypes FAILED
      java.lang.NullPointerException
          at org.apache.orc.impl.PhysicalFsWriter.writeFileMetadata(PhysicalFsWriter.java:415)
          at org.apache.orc.impl.WriterImpl.writeMetadata(WriterImpl.java:578)
          at org.apache.orc.impl.WriterImpl.writeFooter(WriterImpl.java:643)
          at org.apache.orc.impl.WriterImpl.close(WriterImpl.java:735)
          at org.apache.iceberg.orc.OrcFileAppender.close(OrcFileAppender.java:127)
          at org.apache.iceberg.flink.data.TestFlinkOrcReaderWriter.writeAndValidate(TestFlinkOrcReaderWriter.java:61)