fix: Don't set tag used as constraint for update if no tag was considered from registry

[jannfis]

Do not return the original tag if no tag found in a repository was considered for being upgradeable to.

This fixes a scenario with digest update strategy, where a tag to track was specified as constraint but did not exist in the registry. This led to an update of the image to the (non-existing) tag, instead of just ignoring it.

Fixes #217

Signed-off-by: jannfis jann@mistrust.net

[codecov]

Codecov Report

Merging #219 (885ed0b) into master (1d19e6a) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   66.77%   66.75%   -0.03%     
==========================================
  Files          20       20              
  Lines        1574     1573       -1     
==========================================
- Hits         1051     1050       -1     
  Misses        424      424              
  Partials       99       99              

Impacted Files	Coverage Δ
pkg/image/version.go	61.40% <100.00%> (-0.67%)	⬇️

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