SPNEGO.from_cli_arguments: don't ask for password when unrequired

[gpotter2]

• fix bug asking for password when unrequired.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.88%. Comparing base (b810032) to head (8543e7d).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/spnego.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4839      +/-   ##
==========================================
+ Coverage   80.80%   80.88%   +0.08%     
==========================================
  Files         366      368       +2     
  Lines       89626    90244     +618     
==========================================
+ Hits        72418    72992     +574     
- Misses      17208    17252      +44     

Files with missing lines	Coverage Δ
scapy/layers/spnego.py	70.80% <0.00%> (ø)

... and 18 files with indirect coverage changes

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