Fixed issues where time-to-seal was NaN

[AndriiDiachuk]

The new time_to_seal metric was reporting NaN for values. Fixed this issue.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 41.81%. Comparing base (9ac49af) to head (be92cbf).

Files with missing lines	Patch %	Lines
module/metrics/transaction.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            v0.37    #6625      +/-   ##
==========================================
+ Coverage   41.56%   41.81%   +0.25%     
==========================================
  Files        2023     1584     -439     
  Lines      180764   148796   -31968     
==========================================
- Hits        75130    62216   -12914     
+ Misses      99398    81520   -17878     
+ Partials     6236     5060    -1176     

Flag	Coverage Δ
unittests	41.81% <0.00%> (+0.25%)	⬆️

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