return error when creating a zero SigningKey

[conradoplg]

Closes #465

Since VerifyingKey::new() is private, it shouldn't possible to create one from identity, so this addresses the whole issue.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% 🎉

Comparison is base (60d9942) 77.46% compared to head (f8a9197) 77.48%.
Report is 5 commits behind head on main.

❗ Current head f8a9197 differs from pull request most recent head f0bd8fc. Consider uploading reports for the commit f0bd8fc to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #496      +/-   ##
==========================================
+ Coverage   77.46%   77.48%   +0.02%     
==========================================
  Files          30       30              
  Lines        2738     2741       +3     
==========================================
+ Hits         2121     2124       +3     
  Misses        617      617              

Files Changed	Coverage Δ
frost-core/src/signing_key.rs	76.00% <100.00%> (+1.53%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[dconnolly]

💪