Fix modexp exponent EIP-7823 limit check

[chfast]

Fix the incorrect application of EIP-7823 for the modexp exponent length. The issue is caused by the pre-EIP-7823 edge case handling of 0 multiplication complexity when both base and mod have 0 length.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.15%. Comparing base (6926c76) to head (eff73d7).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
test/state/precompiles.cpp	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
- Coverage   87.15%   87.15%   -0.01%     
==========================================
  Files         167      167              
  Lines       24729    24736       +7     
  Branches     4064     4065       +1     
==========================================
+ Hits        21552    21558       +6     
  Misses        538      538              
- Partials     2639     2640       +1     

Flag	Coverage Δ
eest-develop	17.57% <0.00%> (-0.01%)	⬇️
eest-develop-gmp	15.60% <0.00%> (-0.01%)	⬇️
eest-fusaka	12.22% <50.00%> (+<0.01%)	⬆️
eest-legacy	11.11% <0.00%> (-0.01%)	⬇️
eest-legacy-silkpre	17.60% <60.00%> (+<0.01%)	⬆️
eest-static	18.63% <60.00%> (+<0.01%)	⬆️
evmone-unittests	83.76% <80.00%> (+<0.01%)	⬆️

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Components	Coverage Δ
core	93.29% <85.71%> (-0.02%)	⬇️
tooling	88.35% <ø> (ø)
tests	84.13% <100.00%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
test/unittests/precompiles_expmod_test.cpp	95.95% <100.00%> (+0.07%)	⬆️
test/state/precompiles.cpp	94.61% <85.71%> (-0.18%)	⬇️

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