RFR: 6825: JMX defineEventProbes fails when no events in XMLDescriptor

[Jessye Coleman-Shapiro]

This patch makes the agent JMX operation 'defineEventProbes' accept an xmlDecsription with no event tags.  Previously,
the only way you could clear all current transforms was to supply the operation with an empty or null xmlDescription.

Now a file with the following structure will also clear all current transforms:
`<jfragent><events></events></jfragent>`

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Commit messages:
 - Make defineEventProbes accept a descriptor with no events

Changes: https://git.openjdk.java.net/jmc/pull/80/files
 Webrev: https://webrevs.openjdk.java.net/jmc/80/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6825
  Stats: 50 lines in 1 file changed: 27 ins; 16 del; 7 mod
  Patch: https://git.openjdk.java.net/jmc/pull/80.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/80/head:pull/80

PR: https://git.openjdk.java.net/jmc/pull/80