get_cell_ngrams and get_neighbor_cell_ngrams yield nothing when the mention is not tabular (fix #471)

[HiromuHota]

Description of the problems or issues

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

See #471.

Does your pull request fix any issue.

Fix #471.

Description of the proposed changes

get_cell_ngrams yields nothing when the mention is not tabular to align with get_col_ngrams.

Test plan

Change the assertion at test_get_cell_ngrams.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 85.80%. Comparing base (cb24e08) to head (3b29633).
Report is 64 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #504   +/-   ##
=======================================
  Coverage   85.80%   85.80%           
=======================================
  Files          88       88           
  Lines        4571     4573    +2     
  Branches      855      855           
=======================================
+ Hits         3922     3924    +2     
  Misses        464      464           
  Partials      185      185           

Flag	Coverage Δ
unittests	85.80% <100.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
src/fonduer/utils/data_model_utils/tabular.py	100.00% <100.00%> (ø)

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[lukehsiao]

LGTM and seems good to make them consistent behavior.