RFR: 6846: Parser needs to skip events with no metadata [v4]

[Marcus Hirt]

> Also fixing some javadoc errors.

Marcus Hirt has updated the pull request incrementally with one additional commit since the last revision:

  Will re-add these later then

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/128/files
  - new: https://git.openjdk.java.net/jmc/pull/128/files/b5467758..a854c069

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=128&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=128&range=02-03

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jmc/pull/128.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/128/head:pull/128

PR: https://git.openjdk.java.net/jmc/pull/128