fix error formating with nil pointers

[Thykof]

Encountered this while working on massalabs/station-massa-wallet#778

Delivery Quality Checklist

• Breaking Changes in API:
  Does this PR introduce breaking changes in the API?

  • If yes, have you considered making it backward compatible?
  • If backward compatibility is not considered, set the "breaking-change" label.

• Changelog:

  • For bugfix PR, set the "bugfix" label
  • If this change should not appear in changelog, use "ignore-for-changelog" label

• Version Update Handling:
  Have you ensured that the version update by user is handled correctly?

• PR Dependency:
  Does this PR depend on another PR?

  • If yes, is it necessary for the dependency to be released prior to merging this one?

• Documentation:

  • Are any necessary changes made to user-facing documentation?
  • Confirm that API documentation is updated with any relevant changes.
  • Check that README and other documentation files are accurate and current.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4e18860) 25.66% compared to head (9e8cdff) 25.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1235   +/-   ##
=======================================
  Coverage   25.66%   25.66%           
=======================================
  Files          25       25           
  Lines        1535     1535           
=======================================
  Hits          394      394           
  Misses       1128     1128           
  Partials       13       13           

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