tcpip: ignore network with a netmask set to None

[MatthieuDartiailh]

For such netmask, the mask is 32 bits long per psutil docs.

[MatthieuDartiailh]

Closes #474

Can you test @mattjaffe42 ?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 24.46%. Comparing base (09d461d) to head (6db5718).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pyvisa_py/tcpip.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #478   +/-   ##
=======================================
  Coverage   24.46%   24.46%           
=======================================
  Files          23       23           
  Lines        3487     3487           
  Branches      398      398           
=======================================
  Hits          853      853           
  Misses       2629     2629           
  Partials        5        5           

Flag	Coverage Δ
unittests	24.46% <0.00%> (ø)

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[mattjaffe42]

@MatthieuDartiailh tested, and confirmed resolves the error!