DO NOT purge the output location if it has content in SparkGraphComputer

The default logic for SparkGraphComputer is to purge all content if the output location is specified. That is a dangerous operation especially for the output location which contains contents.

https://github.com/apache/tinkerpop/blob/master/spark-gremlin/src/main/java/org/apache/tinkerpop/gremlin/spark/process/computer/SparkGraphComputer.java#L317:L324

The correct behavior is to stop the process if it detects the output location is not empty.