test: Allow "to": null in JSON transaction

[chfast]

Fixes #923.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.15%. Comparing base (d95893c) to head (d3ceaef).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #927   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files         140      140           
  Lines       15796    15805    +9     
=======================================
+ Hits        15030    15039    +9     
  Misses        766      766           

Flag	Coverage Δ
eof_execution_spec_tests	15.86% <27.27%> (-0.01%)	⬇️
ethereum_tests	26.61% <27.27%> (-0.01%)	⬇️
ethereum_tests_silkpre	18.46% <27.27%> (-0.01%)	⬇️
execution_spec_tests	17.94% <27.27%> (-0.01%)	⬇️
unittests	90.27% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
test/statetest/statetest_loader.cpp	96.87% <100.00%> (+0.01%)	⬆️
test/unittests/statetest_loader_tx_test.cpp	100.00% <100.00%> (ø)