fix(digest): check container image info for nil

[piksel]

Fixes #1026

[codecov]

Codecov Report

Merging #1027 (df7d049) into main (7221704) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
+ Coverage   59.63%   59.68%   +0.05%     
==========================================
  Files          22       22              
  Lines        1412     1414       +2     
==========================================
+ Hits          842      844       +2     
  Misses        500      500              
  Partials       70       70              

Impacted Files	Coverage Δ
pkg/registry/digest/digest.go	44.26% <100.00%> (+1.88%)	⬆️

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