fix(yoshi-go): store the sha of the first commit

[bcoe]

If the first commit was of the all type, we were setting its sha to null, this meant we had no sha to base a commit on.

[codecov]

Codecov Report

Merging #523 into master will decrease coverage by 0.44%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   81.67%   81.23%   -0.45%     
==========================================
  Files          42       42              
  Lines        4977     4993      +16     
  Branches      401      406       +5     
==========================================
- Hits         4065     4056       -9     
- Misses        911      935      +24     
- Partials        1        2       +1     

Impacted Files	Coverage Δ
src/releasers/go-yoshi.ts	93.60% <96.15%> (-0.63%)	⬇️
src/util/coerce-option.ts	96.66% <0.00%> (-3.34%)	⬇️
src/github.ts	75.00% <0.00%> (-1.34%)	⬇️
src/conventional-commits.ts	97.15% <0.00%> (-1.22%)	⬇️
src/graphql-to-commits.ts	89.34% <0.00%> (-1.02%)	⬇️
src/release-pr.ts	90.03% <0.00%> (-0.67%)	⬇️
src/releasers/php-yoshi.ts	93.02% <0.00%> (-0.39%)	⬇️
src/releasers/java-yoshi.ts	87.76% <0.00%> (-0.35%)	⬇️

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