Fix unsafe std::vector::data() nullptr assumption for empty vectors

[Copilot]

std::vector::data() is not guaranteed to return nullptr when empty. The PointCloud base class uses raw pointer nullness to signal attribute absence (has_times(), has_points(), etc.), but PointCloudCPU assigns storage.data() unconditionally — so empty vectors can produce non-null pointers, causing has_*() to return true with no actual data.

Fix: Replace all ptr = storage.data() with ptr = storage.empty() ? nullptr : storage.data().

• point_cloud_cpu.cpp — add_times/points/normals/covs/intensities, clone(), load(), aux_attributes
• point_cloud_cpu_funcs.cpp — sample(), voxelgrid_sampling(), aux_attributes
• gaussian_voxelmap_cpu_funcs.cpp — merge_frames()
• point_cloud_gpu.cu — download_points()
• point_cloud_cpu.hpp — add_aux_attribute() template
• intensity_gradients.cpp — inline normal/cov estimation
• incremental_voxelmap_impl.hpp — points assignment

// Before: data() may return non-null for empty vector
this->times = times_storage.data();

// After: explicitly null when empty
this->times = times_storage.empty() ? nullptr : times_storage.data();