Check existence of optional fields during Electrode table population

[khl02007]

Description

The only required fields for nwbfile.add_electrode are location and electrode group (https://pynwb.readthedocs.io/en/stable/pynwb.file.html#pynwb.file.NWBFile.add_electrode). Therefore some NWB files may not other fields such as filtering. Spyglass should check if these exist before trying to populate them; otherwise they should get the default value (e.g. NULL)

Checklist:

• This PR should be accompanied by a release: (yes/no/unsure)
• If release, I have updated the CITATION.cff
• This PR makes edits to table definitions: (yes/no)
• If table edits, I have included an alter snippet for release notes.
• I have updated the CHANGELOG.md with PR number and description.
• I have added/edited docs/notebooks to reflect the changes