Fix: check for None instead of false value in sort_ref_id (current li…

[sharon-chiang]

…ne raises exception if electrode 0 is set to reference)

Description

Please include a summary of the changes and the related issue. Please also
include relevant motivation and context. Please list issues fixed or closed by
This PR.

`/home/sc4712/src/spyglass/src/spyglass/spikesorting/utils.py': Currently li 79 of function get_group_by_shank reads 'if not sort_ref_id', which results in raising an exception if trying to set reference electrode to the 0th electrode.

Alter notes:
if references:
sort_ref_id = references.get(e_group, None)
if sort_ref_id is None: ##### this line was changed to not result in raising an exception if sort_ref_id is set to 0 (the 0th electrode)
raise Exception(
f"electrode group {e_group} not a key in "
+ "references, so cannot set reference"
)

Checklist:

• If this PR should be accompanied by a release, I have updated the CITATION.cff
• If this PR edits table definitions, I have included an alter snippet for release notes.
• If this PR makes changes to position, I ran the relevant tests locally.
• If this PR makes user-facing changes, I have added/edited docs/notebooks to reflect the changes
• I have updated the CHANGELOG.md with PR number and description.