[PWGHF] fixes to D+ inv mass THnSparse for PV contributors

[fjonasALICE]

• added proper handling of PV contributor axis for invMass THnSparse
• In particular, fixed a critical bug which occured when running without storing centrality, without occupancy but with PV contributors. In this case the lines:

     if (storePvContributors) {
        axes.push_back(thnAxisPvContributors);
      }

were missing, so the ThNSparse had not enough axes. This meant that in the filling, the occupancy was treated as a weight. By default, number of pv contributors was -1, so every axis got filled with weight -1 for all quantities when running in this mode.

Changes were validated in local running on HF derived data.

[github-actions]

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