ParquetRecordWriterProvider - Add null check on underlying ParquetWriter

Fixes edge case where it's possible that getDataSize() is called before
the first call to write().

src/main/java/io/confluent/connect/hdfs/parquet/ParquetRecordWriterProvider.java

@@ -56,6 +56,9 @@ public RecordWriter getRecordWriter(HdfsSinkConnectorConfig conf, String filenam
     return new SizeAwareRecordWriter() {
       @Override
       public long getDataSize() {
+        if (writer == null) {
+          return 0;
+        }
         return writer.getDataSize();
       }