Blocked: Fix nox when HOME not set

[TomDufall]

When a HOME environment variable isn't set, nox --list currently fails due to trying to cast None to a Path. I noticed this when trying to run tests on my Windows PC.

This PR adds a check before casting the env var to a Path. Having None as a value should be acceptable - CARTOPY_CACHE_DIR is later checked for None values.

[TomDufall]

nox > python -m iris_grib.tests.runner --default-tests --unit-tests --integration-tests
Traceback (most recent call last):
File "/tmp/cirrus-ci-build/.nox/tests-iris-conda-forge/lib/python3.6/runpy.py", line 183, in _run_module_as_main
mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
File "/tmp/cirrus-ci-build/.nox/tests-iris-conda-forge/lib/python3.6/runpy.py", line 109, in _get_module_details
import(pkg_name)
File "/tmp/cirrus-ci-build/iris_grib/init.py", line 28, in
from iris.util import _array_slice_ifempty
ImportError: cannot import name '_array_slice_ifempty'

[TomDufall]

The function has recently been removed in Iris here.

The PR says that this is a um-data-specific workaround that is no longer required - I assume that means it can also be removed here? I don't know the answer to this, but I'll raise a PR that solves the technical issue, assuming it is fine to do so, and someone who knows the context can decide if that's the right thing to do.

[TomDufall]

Closing this PR as dependencies don't support Windows and Linux should generally have a $HOME. The fixes to master will continue in #270