RFR: 8255992: JFR EventWriter does not use first string from StringPool with id 0

[Jie Kang]

This allows string pool entry with id 0 to be written in short form for the event data.

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Commit messages:
 - 8255992: JFR EventWriter does not use first string from StringPool with id 0

Changes: https://git.openjdk.java.net/jdk/pull/1097/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1097&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255992
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1097.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1097/head:pull/1097

PR: https://git.openjdk.java.net/jdk/pull/1097