Fix assign_reg_io for ndarray input

[ethanbb]

I ran into this when trying to use registration_wrapper by itself in another project. registration_wrapper and its subroutine assign_reg_io are documented to take either BinaryFiles or np.ndarrays, but currently passing an ndarray gives the error "The truth value of an array with more than one element is ambiguous" due to trying to test the truth value of the input directly. This is fixed by changing each check to is None or is not None.

[StephenThornquist]

Wow synchronicity #1227

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.32%. Comparing base (be8830d) to head (12b5123).
⚠️ Report is 228 commits behind head on main.

Files with missing lines	Patch %	Lines
suite2p/registration/register.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1226      +/-   ##
==========================================
+ Coverage   55.21%   58.32%   +3.10%     
==========================================
  Files          43       47       +4     
  Lines        5055     4434     -621     
==========================================
- Hits         2791     2586     -205     
+ Misses       2264     1848     -416     

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