Fix GeneratorPythia8 construction

[sawenzel]

Fixes smaller issues introduced with the hybrid generator refactoring:

• Make sure that internal configuration object is never null; In fact, it does not need to be a pointer.

• Make sure the internal configuration struct is initialized from the GeneratorPythia8Param configurable when using the default constructor.

This commit fixes an issue/segfault when running the following command

o2-sim-dpl-eventgen --generator external --nEvents 200 --aggregate-timeframe 10000 --configFile ${O2DPG_ROOT}/MC/config/ALICE3/ini/pythia8_pp_136tev.ini -b

(which defaults constructs a Pythia8 generator)

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
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[alibuild]

Error while checking build/O2/fullCI for c8289a5 at 2024-11-21 17:59:

## sw/BUILD/O2-latest/log
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
c++: error: unrecognized command-line option '--rtlib=compiler-rt'

Full log here.