[PWGEM] Fix clang-tidy warning and errors in more photon meson code

[mhemmer-cern]

• Fix clang-tidy errors and warnings in DalitzEE and EMCal related code

[github-actions]

O2 linter results: ❌ 11 errors, ⚠️ 88 warnings, 🔕 18 disabled

[alibuild]

Auto-approving on behalf of @mhemmer-cern.

[alibuild]

Error while checking build/O2Physics/code-check for d347b80 at 2026-07-14 21:45:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 6 errors and 0 warnings.'
Found 6 errors and 0 warnings.
++ [[ 6 -gt 0 ]]
++ cat /sw/BUILD/2e9f14a70cd3190494c6989f5dc1a092e5d37822/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/TableProducer/skimmerGammaCalo.cxx:261:11: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
PWGEM/PhotonMeson/TableProducer/skimmerGammaCalo.cxx:271:11: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
PWGEM/PhotonMeson/TableProducer/skimmerPrimaryElectronFromDalitzEE.cxx:513:44: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
PWGEM/PhotonMeson/TableProducer/skimmerPrimaryElectronFromDalitzEE.cxx:513:91: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
PWGEM/PhotonMeson/TableProducer/skimmerPrimaryElectronFromDalitzEE.cxx:615:44: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
PWGEM/PhotonMeson/TableProducer/skimmerPrimaryElectronFromDalitzEE.cxx:615:91: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 6 -gt 0 ]]
++ exit 1
--

Full log here.