Fix the bug for computing quality metrics in spikesorting v0

[sytseng]

Description

I added a bug fix for spikesorting.v0.spikesorting_curation.QualityMetrics._compute_metric described in #1202 to make sure num_spikes is included when num_spikes < min_spikes.

Checklist:

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

[CBroz1]

Thanks @sytseng ! I edited your description to add the template checklist. Looks like the only thing missing is adding a line-item to the CHANGELOG with this PR number, 1212

[khl02007]

I may be wrong about this since it's been a while since I wrote this (please correct me if that is the case). It seems we need special handling of this case (i.e. number of spikes for a unit is below min_spikes) only for computing nn_isolation and nn_overlap metrics. So instead of adding not is_num_spikes, how about changing to code to check for only those two conditions and have all the others go to ? That way we won't have to fix this again when we compute another metric (like snr, isi_violation, peak_offset etc)