Allow CurationV1 to save no-spikes case

[CBroz1]

Description

Adds fallback for no-spikes case to V1 save logic

Checklist:

• N/a. If this PR should be accompanied by a release, I have updated the CITATION.cff
• N/a. If this PR edits table definitions, I have included an alter snippet for release notes.
• N/a. If this PR makes changes to position, I ran the relevant tests locally.
• N/a. 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.

[Copilot]

Pull request overview

Adds a fallback path in CurationV1 NWB save logic to support “no spikes / empty units” sortings, ensuring an empty Units table can still be written and referenced downstream.

Changes:

• Handle missing spike_times column by treating the sorting as empty and skipping merge application when there are no units.
• Create an explicit empty pynwb.misc.Units table in the output analysis NWB when there are no unit IDs to write.
• Add tests covering the no-spikes save path and verifying an NWB with an empty units table is readable; add a changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/spyglass/spikesorting/v1/curation.py	Adds no-spikes fallback for missing spike_times and writes an empty Units table when appropriate.
tests/spikesorting/v1/test_curation.py	Adds tests/fixtures to exercise no-spikes behavior and validate empty-units NWB readability.
CHANGELOG.md	Notes the no-spikes save support for CurationV1 and adjusts an existing entry’s formatting.

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