Add nullable values specifiers for add_electrodes

[h-mayorquin]

Has #984 as base. This drops the int casting for the electrrodes table as discussed in #964.

The important thing is that it drops a bug where data is lost when converting int64 to float.

I will make a similar one for the units table.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.43%. Comparing base (fc6ae26) to head (34870ff).
Report is 2 commits behind head on main.

Files	Patch %	Lines
...c/neuroconv/tools/spikeinterface/spikeinterface.py	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #985      +/-   ##
==========================================
+ Coverage   90.63%   91.43%   +0.80%     
==========================================
  Files         127      127              
  Lines        7525     7540      +15     
==========================================
+ Hits         6820     6894      +74     
+ Misses        705      646      -59     

Flag	Coverage Δ
unittests	91.43% <95.45%> (+0.80%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...c/neuroconv/tools/spikeinterface/spikeinterface.py	95.12% <95.45%> (+11.99%)	⬆️