Fix output HDF5 bugs when there are no objects in the HaloMerger test problem

[hsinhaoHHuang]

Issue

When OPT__INIT = 3 is adopted in the HaloMerger test problem, there will be an error

Output_DumpData_Total_HDF5 (DumpID = 0)     ...
********************************************************************************
ERROR : HDF5_Output_t structure must not be empty !!
        Rank <0>, file <Output/Output_DumpData_Total_HDF5.cpp>, line <4057>, function <GetCompound_General>
********************************************************************************

This is because HaloMerger_Halo_Num, HaloMerger_ParCloud_Num, and HaloMerger_Soliton_Num are all 0 in this case, and the output structure becomes empty.

Solution

Set Output_HDF5_UserPara_Ptr to NULL when HaloMerger_Halo_Num, HaloMerger_ParCloud_Num, and HaloMerger_Soliton_Num are all 0.