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 42.83%. Comparing base (a80d4c5) to head (40d5c3e).
Report is 17 commits behind head on master.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6624      +/-   ##
==========================================
+ Coverage   41.21%   42.83%   +1.62%     
==========================================
  Files        2054     1621     -433     
  Lines      182340   148295   -34045     
==========================================
- Hits        75154    63526   -11628     
+ Misses     100886    79307   -21579     
+ Partials     6300     5462     -838     

Flag	Coverage Δ
unittests	42.83% <0.00%> (+1.62%)	⬆️

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