fix(registry): ignore empty challenge fields

[piksel]

The GetAuthURL method splits challenge response header fields by = and then stores the key and value in a map. If an empty field (or a key without a value) is encountered, this causes a "index out of range" panic.

This PR replaces the unchecked strings.Split with strings.Cut, and skips any fields that does not contain a value.

Fixes #1624.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 ⚠️

Comparison is base (9d6b008) 66.91% compared to head (d96481d) 66.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1626      +/-   ##
==========================================
- Coverage   66.91%   66.89%   -0.03%     
==========================================
  Files          25       25              
  Lines        2376     2374       -2     
==========================================
- Hits         1590     1588       -2     
  Misses        685      685              
  Partials      101      101              

Impacted Files	Coverage Δ
pkg/registry/auth/auth.go	42.44% <100.00%> (-0.82%)	⬇️

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