change auth behavior to match upstream on unknown/empty user - use null auth

[aviramha]

change auth behavior to match upstream on unknown/empty user - use null auth
Closes #1594
Should help metalbear-co/mirrord#2803

[clux]

Thank you! Looks sensible to me. Good find.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.2%. Comparing base (7920fc3) to head (3f0b0d6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kube-client/src/config/file_loader.rs	0.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1595     +/-   ##
=======================================
- Coverage   75.3%   75.2%   -0.0%     
=======================================
  Files         82      82             
  Lines       7331    7334      +3     
=======================================
- Hits        5515    5514      -1     
- Misses      1816    1820      +4     

Files with missing lines	Coverage Δ
kube-client/src/config/mod.rs	45.0% <ø> (ø)
kube-client/src/config/file_loader.rs	78.9% <0.0%> (-6.8%)	⬇️

[aviramha]

Seems that nightly fails are not related to the changes.

[clux]

Yeah, I don't know what's up with nightly here. But it's not worth worrying about atm I think.