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

[Johan Sjölen]

On Tue, 17 Jan 2023 11:26:12 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

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

Hi @mgronlun, the "fix" comments are there because I intend to revert the macros to regular old `NULL`. I'll copy paste a more informative comment next time :).

Also, the goal for comments is to use "null" unless it's directly used in a code expression. Examples:

```c++
// This function returns null
void* ret_null();
// This function returns true if x == nullptr
bool is_nullptr(void* x);

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