kinematics_correlations.py: ak.count -> ak.num where needed

Fixes failure seen in https://eicweb.phy.anl.gov/EIC/benchmarks/physics_benchmarks/-/jobs/1755651

>>> import awkward as ak
>>> ak.count([[1], [2]], axis=0)
<Array [2] type='1 * int64'>
>>> ak.num([[1], [2]], axis=0)
2

requested review from @wdconinc

assigned to @veprbl

Prevents script from failing when input has zero entries. But doesn't fix why there are zero entries to begin with. We should fail out explicitly when there are no entries, whether here or at an earlier stage.

approved this merge request

added 2 commits

• bdb21395 - 1 commit from branch master
• 50f0d5d2 - kinematics_correlations.py: ak.count -> ak.num where needed

Compare with previous version

enabled an automatic merge when the pipeline for 50f0d5d2 succeeds

aborted the automatic merge because target branch was updated

added 6 commits

• 50f0d5d2...28424bd6 - 5 commits from branch master
• bbc92dc4 - kinematics_correlations.py: ak.count -> ak.num where needed

Compare with previous version

added 11 commits

• bbc92dc4...8ce60849 - 10 commits from branch master
• 57ca12e4 - kinematics_correlations.py: ak.count -> ak.num where needed

Compare with previous version