RFR: 8048: Agent throws exceptions on missing or empty descriptions

[Ralf Schmelter]

The agent throws exceptions when the description of an event or value is empty or missing. This change just omits the Description annotation for the generated event classes in this case.

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Commit messages:
 - Handle empty or missing descriptions gracefully

Changes: https://git.openjdk.org/jmc/pull/473/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=473&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8048
  Stats: 10 lines in 1 file changed: 4 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jmc/pull/473.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/473/head:pull/473

PR: https://git.openjdk.org/jmc/pull/473