Add nullables to candidate_subclass()

[HiromuHota]

Description of the problems or issues

Is your pull request related to a problem? Please describe.

See #496.

Does your pull request fix any issue.

Fix #496.

Description of the proposed changes

Add nullables to candidate_subclass()

Test plan

Added test_candidates.py::test_candidate_with_nullable_mentions

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the CHANGELOG.rst accordingly.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.80%. Comparing base (afe12b0) to head (2ce00a9).
Report is 68 commits behind head on master.

Files with missing lines	Patch %	Lines
src/fonduer/candidates/models/candidate.py	60.00%	1 Missing and 1 partial ⚠️
src/fonduer/candidates/candidates.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
+ Coverage   85.76%   85.80%   +0.03%     
==========================================
  Files          88       88              
  Lines        4567     4571       +4     
  Branches      853      855       +2     
==========================================
+ Hits         3917     3922       +5     
+ Misses        466      464       -2     
- Partials      184      185       +1     

Flag	Coverage Δ
unittests	85.80% <62.50%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/fonduer/utils/utils.py	79.31% <ø> (ø)
src/fonduer/candidates/candidates.py	86.90% <66.66%> (ø)
src/fonduer/candidates/models/candidate.py	74.60% <60.00%> (-1.67%)	⬇️

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[HiromuHota]

Oh, "black" has been updated just a few hours ago.