fix(worker): Allow commit summaries (snapshot API) with no files

[nellh]

The 404 here didn't really make sense because it would fail if get_tree returned a tree with no files. That's a valid tree, so it should return the 200 case.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.82%. Comparing base (79b32ba) to head (598b5b2).

Files	Patch %	Lines
...ices/datalad/datalad_service/handlers/snapshots.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3086       +/-   ##
===========================================
+ Coverage   42.36%   72.82%   +30.46%     
===========================================
  Files         595       43      -552     
  Lines       37921     1608    -36313     
  Branches     1096        0     -1096     
===========================================
- Hits        16064     1171    -14893     
+ Misses      21645      437    -21208     
+ Partials      212        0      -212     

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