Remove unnecessary null pointer checks

[An extra null pointer check is not needed in functions](https://isocpp.org/wiki/faq/freestore-mgmt#delete-handles-null "Do I need to check for null before delete p?") like the following.
- [DensityOp](https://github.com/dreamworksanimation/openvdb/blob/18568814988362db3b6d661491a8179e6cec71de/openvdb_houdini/houdini/SOP_OpenVDB_Rasterize_Points.cc#L1547 "Destructor for DensityOp")
- [OpFactory::setObsoleteParms](https://github.com/dreamworksanimation/openvdb/blob/a7a1abde7c955d5a83acac2c82341497d73576c2/openvdb_houdini/houdini/ParmFactory.cc#L1232)
- [TestDenseSparseTools:tearDown](https://github.com/dreamworksanimation/openvdb/blob/a7a1abde7c955d5a83acac2c82341497d73576c2/openvdb/unittest/TestDenseSparseTools.cc#L44)

Start by reviewing the three linked locations: SOP_OpenVDB_Rasterize_Points.cc, ParmFactory.cc, and TestDenseSparseTools.cc. Compare each null check with the linked C++ guidance, remove only the unnecessary checks, and run the TestDenseSparseTools test along with relevant project tests to confirm the cleanup is safe.

Written by the indexing model from the issue text.