Fixed PKI signature verification fallback on hardcoded keys

[apaillier-ledger]

Description

#946 broke it

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.02%. Comparing base (e03930b) to head (3dd21a5).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #979   +/-   ##
========================================
  Coverage    55.02%   55.02%           
========================================
  Files           16       16           
  Lines         1592     1592           
  Branches       201      201           
========================================
  Hits           876      876           
  Misses         712      712           
  Partials         4        4           

Flag	Coverage Δ
unittests	55.02% <ø> (ø)

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