RFR: JDK-8301225: Replace NULL with nullptr in share/gc/shenandoah/

[Johan Sjölen]

Hi, this PR changes all occurrences of NULL to nullptr for the subdirectory share/gc/shenandoah/. Unfortunately the script that does the change isn't perfect, and so we
need to comb through these manually to make sure nothing has gone wrong. I also review these changes but things slip past my eyes sometimes.

Here are some typical things to look out for:

1. No changes but copyright header changed (probably because I reverted some changes but forgot the copyright).
2. Macros having their NULL changed to nullptr, these are added to the script when I find them. They should be NULL.
3. nullptr in comments and logs. We try to use lower case "null" in these cases as it reads better. An exception is made when code expressions are in a comment.

An example of this:

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


Note how `nullptr` participates in a code expression here, we really are talking about the specific value `nullptr`.

Thanks!

-------------

Commit messages:
 - Manual fixes
 - Merge remote-tracking branch 'origin/master' into JDK-8301225
 -  Replace NULL with nullptr in share/gc/shenandoah/

Changes: https://git.openjdk.org/jdk/pull/12251/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12251&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301225
  Stats: 536 lines in 60 files changed: 0 ins; 0 del; 536 mod
  Patch: https://git.openjdk.org/jdk/pull/12251.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12251/head:pull/12251

PR: https://git.openjdk.org/jdk/pull/12251