core/types: fix and test handling of faulty nil-returning signer

[ucwong]

No description provided.

[codecov]

Codecov Report

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

Comparison is base (9849c6f) 52.82% compared to head (367684b) 52.83%.
Report is 467 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1900   +/-   ##
=======================================
  Coverage   52.82%   52.83%           
=======================================
  Files         494      494           
  Lines       60543    60545    +2     
=======================================
+ Hits        31984    31989    +5     
+ Misses      26114    26109    -5     
- Partials     2445     2447    +2     

Files	Coverage Δ
core/types/transaction.go	63.53% <0.00%> (-0.71%)	⬇️

... and 11 files with indirect coverage changes