timeout certificate can be nil

[illia-malachyn]

fix for onflow/flow-go#6571 (comment)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 57.03%. Comparing base (9f84549) to head (1221b62).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
access/grpc/convert/convert.go	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #790      +/-   ##
==========================================
- Coverage   57.04%   57.03%   -0.01%     
==========================================
  Files          35       35              
  Lines        7391     7392       +1     
==========================================
  Hits         4216     4216              
- Misses       2673     2674       +1     
  Partials      502      502              

Flag	Coverage Δ
unittests	57.03% <28.57%> (-0.01%)	⬇️

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[illia-malachyn]

cc @turbolent @peterargue @janezpodhostnik