RFR: JDK-8300245: Replace NULL with nullptr in share/jfr/ [v2]

[Johan Sjölen]

> Do the conversion in the share/jfr/ sub-directory and all of its files.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Fix two faulty NULL_STRING misses

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12034/files
  - new: https://git.openjdk.org/jdk/pull/12034/files/4ae78ad9..a8a4c75b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12034&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12034&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12034.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12034/head:pull/12034

PR: https://git.openjdk.org/jdk/pull/12034