fix core when extra_data is nullptr

[LucasLi1024]

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (3695845) 25.80% compared to head (d81ee68) 25.80%.

Files	Patch %	Lines
bcos-c-sdk/bcos_sdk_c_uti_tx_struct.cpp	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           release-3.6.0     #213   +/-   ##
==============================================
  Coverage          25.80%   25.80%           
==============================================
  Files                  6        6           
  Lines               1763     1763           
  Branches            1078     1078           
==============================================
  Hits                 455      455           
  Misses               937      937           
  Partials             371      371           

Flag	Coverage Δ
unittests	25.80% <0.00%> (ø)

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