[EMCAL-688] ClusterFactory: remove std::optional

[mhemmer-cern]

• std::optional creates copies, potentially slowing down the code and increase memory usage. Instead we just use a gsl::span here and check if it is empty or not.

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
To request your PR to be included in production software, please add the corresponding labels called "async-" to your PR. Add the labels directly (if you have the permissions) or add a comment of the form (note that labels are separated by a ",")

+async-label <label1>, <label2>, !<label3> ...

This will add <label1> and <label2> and removes <label3>.

The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0
async-2024-PbPb-apass1
async-2024-ppRef-apass1
async-2024-PbPb-apass2
async-2023-PbPb-apass5

[mhemmer-cern]

Needs to be merged after AliceO2Group/O2Physics#11303

[alibuild]

Error while checking build/O2/fullCI_slc9 for 2739249 at 2025-05-22 21:07:

## sw/BUILD/O2Physics-latest/log
/sw/SOURCES/O2Physics/14314-slc9_x86-64/0/PWGJE/TableProducer/emcalCorrectionTask.cxx:618:91: error: cannot convert 'std::optional<const gsl::span<o2::emcal::CellLabel> >' to 'gsl::span<const o2::emcal::CellLabel>'
ninja: build stopped: subcommand failed.


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
++ GRERR=1
++ [[ 1 == 0 ]]
++ mkdir -p /sw/INSTALLROOT/17533e9248663bc9bcb79046939d1fd190009257/slc9_x86-64/o2checkcode/1.0-local54/etc/modulefiles
++ cat
--

Full log here.

[mhemmer-cern]

O2Physics PR just got merged. Should be able to merge soon.