commit summary fails on 'author_email' being NULL

[dray92]

Before change:

devzero@ngvzugqq mergestat-lite (main) $ make .build/mergestat && .build/mergestat -r ../services/ summarize commits --verbose
-- building .build/mergestat
-- built .build/mergestat
May 15 18:03:02 INF opening repo path=../services/
May 15 18:03:48 ERR sql: Scan error on column index 1, name "author_email": converting NULL to string is unsupported

After change:

devzero@ngvzugqq mergestat-lite (main) $ make .build/mergestat && .build/mergestat -r ../services/ summarize commits --verbose
-- building .build/mergestat
-- built .build/mergestat
May 15 18:06:50 INF opening repo path=../services/
wunderwuzzi23         88        3.49%      49        27,351      14,622      3 years ago (2021-05-05)   

[codecov]

Codecov Report

Merging #381 (6e04d1b) into main (316c406) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #381   +/-   ##
=======================================
  Coverage   70.92%   70.92%           
=======================================
  Files          57       57           
  Lines        4461     4461           
=======================================
  Hits         3164     3164           
  Misses        928      928           
  Partials      369      369           

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