pvNVIDIAIndeX: fix `clang-tidy` `modernize-use-nullptr` lints

Plugins/pvNVIDIAIndeX/src/vtknvindex_cluster_properties.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_config_settings.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_forwarding_logger.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_host_properties.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_host_properties.h

Plugins/pvNVIDIAIndeX/src/vtknvindex_instance.cxx

.../pvNVIDIAIndeX/src/vtknvindex_irregular_volume_mapper.cxx

...AIndeX/src/vtknvindex_irregular_volume_representation.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_opengl_app_buffer.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_opengl_app_buffer.h

...vNVIDIAIndeX/src/vtknvindex_regular_volume_properties.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_representation.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_utilities.h

Plugins/pvNVIDIAIndeX/src/vtknvindex_volume_compute.cxx

Plugins/pvNVIDIAIndeX/src/vtknvindex_volumemapper.cxx