Fix panic for DSSE canonicalization

[haydentherapper]

Handles if the array of signatures contains missing data.

Summary

Release Note

Documentation

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (7788eb3) 66.40% compared to head (2e08c53) 47.60%.

Files	Patch %	Lines
pkg/types/dsse/v0.0.1/entry.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1923       +/-   ##
===========================================
- Coverage   66.40%   47.60%   -18.81%     
===========================================
  Files          92       92               
  Lines        9193     9197        +4     
===========================================
- Hits         6105     4378     -1727     
- Misses       2345     3943     +1598     
- Partials      743      876      +133     

Flag	Coverage Δ
e2etests	47.60% <25.00%> (-0.01%)	⬇️
unittests	?

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