[PWGCF] Change and fix according to code check

[meiALICE]

Fix includes.

Fix a logical bug.

Replacement of all reinterpret_cast.

More use of nullptr.

Other fixes and improvements according to code-check result.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 0 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for b6bdff6 at 2026-09-07 13:22:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 6 errors and 6 warnings.'
Found 6 errors and 6 warnings.
++ [[ 6 -gt 0 ]]
++ cat /sw/BUILD/6b4e71f96b4554c4d7171c56b09ed9ab902b3106/O2Physics-code-check/errors.txt
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:32:1: error: included header TColor.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:41:1: error: included header TStyle.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:127:8: error: constructor does not initialize these fields: ccdb [cppcoreguidelines-pro-type-member-init,-warnings-as-errors]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:406:5: error: switching on non-enum value without default case may not cover all cases [bugprone-switch-missing-default-case,-warnings-as-errors]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:422:5: error: switching on non-enum value without default case may not cover all cases [bugprone-switch-missing-default-case,-warnings-as-errors]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:559:5: error: switching on non-enum value without default case may not cover all cases [bugprone-switch-missing-default-case,-warnings-as-errors]
++ [[ 6 -gt 0 ]]
++ cat /sw/BUILD/6b4e71f96b4554c4d7171c56b09ed9ab902b3106/O2Physics-code-check/warnings.txt
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:173:5: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:180:5: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:181:5: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:182:5: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:188:5: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGCF/MultiparticleCorrelations/Tasks/multiparticleCorrelationsMei.cxx:244:12: warning: use nullptr [modernize-use-nullptr]
++ [[ 6 -gt 0 ]]
++ exit 1
--

Full log here.