avoid failing when lgetxattr has no data to give

[hallyn]

No description provided.

[hallyn]

See project-stacker/stacker#586

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (9da0fd4) 73.48% compared to head (ad8ce08) 46.41%.

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #526       +/-   ##
===========================================
- Coverage   73.48%   46.41%   -27.08%     
===========================================
  Files          60       60               
  Lines        4888     4891        +3     
===========================================
- Hits         3592     2270     -1322     
- Misses        937     2353     +1416     
+ Partials      359      268       -91     

Files	Coverage Δ
oci/layer/tar_generate.go	42.76% <0.00%> (-27.71%)	⬇️

... and 32 files with indirect coverage changes

[tych0]

if you rebase hopefully CI will pass.

[hallyn]

Thanks. "here goes"