add check for existence of IntracellularElectrode.cell_id

[bendichter]

fix #103

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• Is your code contribution compliant with black format? If not, simply pip install black and run black . inside your fork.
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov-commenter]

Codecov Report

Merging #256 (91c400b) into dev (0fafb09) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #256      +/-   ##
==========================================
+ Coverage   94.28%   94.32%   +0.04%     
==========================================
  Files          18       19       +1     
  Lines         962      969       +7     
==========================================
+ Hits          907      914       +7     
  Misses         55       55              

Flag	Coverage Δ
unittests	94.32% <100.00%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
nwbinspector/__init__.py	100.00% <100.00%> (ø)
nwbinspector/checks/icephys.py	100.00% <100.00%> (ø)