Use correct dtype for empty box samples

[uri-granta]

Related issue(s)/PRs:

Summary

sample_sobol and sample_halton should return an empty sample with the correct dtype for num_samples = 0

Fully backwards compatible: yes / no

PR checklist

• [ ] The quality checks are all passing
• [ ] The bug case / new feature is covered by tests
• [ ] Any new features are well-documented (in docstrings or notebooks)