[SPARK-15737][CORE] fix jetty warning

## What changes were proposed in this pull request?

After upgrading Jetty to 9.2, we always see "WARN org.eclipse.jetty.server.handler.AbstractHandler: No Server set for org.eclipse.jetty.server.handler.ErrorHandler" while running any test cases.

This PR will fix it.

## How was this patch tested?

The existing test cases will cover it.

Author: bomeng <bmeng@us.ibm.com>

Closes #13475 from bomeng/SPARK-15737.