[clang-tidy] Use nullptr explicitly and initialize directly.

[darnuria]

Short description

An other clang-tidy in pdns_utils for nullptr + initialize. Moved the comments upper to avoid depassing 80char limit.

No problem if a rollup is justified of #11966 #11910 #11957, did separated one to avoid a big commit/pr.

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [x] compiled this code
• [x] tested this code unit tested same issue deadlock like in #11966
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master

[darnuria]

Thanks for review the extra parentheses comes from the clang-tidy suggestion will push a change right now. Changes adressed in d0ffeed

[darnuria]

Squashed and rebased on top of master. Note failed in CI after rebase (raw log link failed to check sum-up in actions) https://pipelines.actions.githubusercontent.com/serviceHosts/042df3b5-d696-43c5-83a4-124a3267b26e/_apis/pipelines/1/runs/33593/signedlogcontent/552?urlExpires=2022-09-25T01%3A42%3A52.5525298Z&urlSigningMethod=HMACV1&urlSignature=MD6vg9T14l91rcHCpjxfuZ2B6wXK4MfHVc04JyB%2FDs4%3D

[darnuria]

Same did a rebase against master and bump.