Fix illegal instruction on osx-64 when returning 0 length arrays

[joaander]

Description

Replace &vector[0] with vector->data(). data() is undefined for 0 length vectors, so also add in a separate code path to return numpy managed arrays when they are 0 length.

Motivation and context

I don't understand why this didn't cause problems sooner. The 4.8.0 builds fail with an illegal instruction on the osx-64 platform: https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=980051&view=logs&j=9c5ef928-2cd6-52e5-dbe6-9d173a7d951b&t=20c71c51-4b27-578b-485d-06ade2de1d00&l=6682

How has this been tested?

I applied this diff as a patch in a test PR and the build now passes:
https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=980708&view=logs&j=9c5ef928-2cd6-52e5-dbe6-9d173a7d951b&t=20c71c51-4b27-578b-485d-06ade2de1d00

Change log

Fixed:

* Prevent illegal instruction when accessing 0 length snapshot arrays
  (`#1846 <https://github.com/glotzerlab/hoomd-blue/pull/1846>`__)

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.