Avoid setting p0 to None during assignment

[faridyagubbayli]

Fixed #468

Not sure why we even have a logic to assign p0 to None. Could not find any reason.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.12%. Comparing base (36b7e9a) to head (e7e5e8a).

Files with missing lines	Patch %	Lines
kwave/ksource.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #543   +/-   ##
=======================================
  Coverage   73.11%   73.12%           
=======================================
  Files          46       46           
  Lines        6830     6831    +1     
  Branches     1308     1308           
=======================================
+ Hits         4994     4995    +1     
  Misses       1286     1286           
  Partials      550      550           

Flag	Coverage Δ
3.10	73.12% <50.00%> (+<0.01%)	⬆️
3.11	73.12% <50.00%> (+<0.01%)	⬆️
3.12	73.12% <50.00%> (+<0.01%)	⬆️
3.13	73.12% <50.00%> (+<0.01%)	⬆️
macos-latest	73.09% <50.00%> (+<0.01%)	⬆️
ubuntu-latest	73.09% <50.00%> (+<0.01%)	⬆️
windows-latest	73.10% <50.00%> (+<0.01%)	⬆️

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