Allow default value of None with enum docval

[rly]

Motivation

Fix #408

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #409 into dev will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #409   +/-   ##
=======================================
  Coverage   79.12%   79.12%           
=======================================
  Files          33       33           
  Lines        6921     6921           
  Branches     1518     1518           
=======================================
  Hits         5476     5476           
  Misses       1058     1058           
  Partials      387      387           

Impacted Files	Coverage Δ
src/hdmf/utils.py	96.21% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 574df03...4582d4c. Read the comment docs.