fix(events): fix initialization on existing event storage

[glevco]

Motivation

In v0.60.0 the signal_bits field was added to event's TxData, but that breaks when initializing with an existing event storage. That happened when trying to update the wallet service node.

Acceptance Criteria

• Make event's TxData.signal_bits nullable.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.92%. Comparing base (ae56108) to head (d0875e9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1038      +/-   ##
==========================================
+ Coverage   84.87%   84.92%   +0.04%     
==========================================
  Files         297      297              
  Lines       22910    22910              
  Branches     3464     3464              
==========================================
+ Hits        19446    19457      +11     
+ Misses       2775     2768       -7     
+ Partials      689      685       -4     

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