[fix] null values are computed preemptively without first checking whether they will actually be needed

[MGAMZ]

This is a follow-up PR from #1624 and #1629 .

This PR delays the computaion of null_value in spikeinterface module. This can avoid the error of being unable to obtain the default null value when null values are not needed.

Note that is PR requires further discussion combining #1624 and #1629 before we determine to merge it.

[Copilot]

Pull request overview

This PR optimizes the computation of null values by deferring their calculation until it's determined that rows will actually be added to NWB tables. This prevents errors when default null values cannot be obtained but aren't needed because no new rows are being added.

Key Changes:

• Moved null value computation in add_electrodes_to_nwbfile to occur only when channel_ids_to_add is non-empty
• Moved null value computation in _add_units_table_to_nwbfile to occur only when rows_to_add is non-empty

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.48%. Comparing base (b3e6463) to head (51520d2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1633   +/-   ##
=======================================
  Coverage   87.48%   87.48%           
=======================================
  Files         159      159           
  Lines       11362    11364    +2     
=======================================
+ Hits         9940     9942    +2     
  Misses       1422     1422           

Flag	Coverage Δ
unittests	87.48% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...c/neuroconv/tools/spikeinterface/spikeinterface.py	80.00% <100.00%> (+0.04%)	⬆️

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