[elixir] bug: allow empty datasets to be passed into dmap feed update

[grejdi-mbta]

When there is no update to the feed, the worker will pass an empty list of datasets to the update function of DMAP feed. This PR handles this case, so max_by/3 doesn't error out.

[github-actions]

Coverage of commit 56ab37b

Summary coverage rate:
  lines......: 90.3% (380 of 421 lines)
  functions..: 70.1% (157 of 224 functions)
  branches...: no data found

Files changed coverage rate:
                                                           |Lines       |Functions  |Branches    
  Filename                                                 |Rate     Num|Rate    Num|Rate     Num
  ===============================================================================================
  lib/ex_cubic_ingestion/schema/cubic_dmap_feed.ex         | 100%     10| 100%    11|    -      0

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[paulswartz]

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