NCHS fix removing None files

[eujing]

Description

When doing archive diffing at the daily level and the daily_cache has actually been populated, we may get empty diffs for files that did not change, and these return a None instead of a file path.

The current code might try to remove these None objects as so:

Traceback (most recent call last):
  File "/home/indicators/.pyenv/versions/3.8.2/lib/python3.8/runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/indicators/.pyenv/versions/3.8.2/lib/python3.8/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/indicators/runtime/nchs_mortality/delphi_nchs_mortality/__main__.py", line 11, in <module>
    run_module()  # pragma: no cover
  File "/home/indicators/runtime/nchs_mortality/delphi_nchs_mortality/run.py", line 87, in run_module
    arch_diffs(params, daily_arch_diff)
  File "/home/indicators/runtime/nchs_mortality/delphi_nchs_mortality/archive_diffs.py", line 82, in arch_diffs
    remove(diff_file)
TypeError: remove: path should be string, bytes or os.PathLike, not NoneType

This PR updates the tests to make sure this case gets tested, and updates the code to ensure we do not try to remove None objects.

Changelog

Itemize code/test/documentation changes and files added/removed.

• delphi_nchs_mortality/archive_diffs.py: Check for None diff files.
• tests/conftest.py: Populate the test daily_cache directory with some files to test empty diffs.
• tests/test_data/weekly_202025_state_wip_deaths_covid_incidence_prop.csv: File that should have no diff from what is generated.

Fixes

• No issue created for this yet.

[jingjtang]

lgtm