Skip advertisements that have no metadata

[gammazero]

If an ad has no entries and no metadata, then it used only to update provider addresses and is not an error.

If an ad has entries but no metadata, then an error is logged an the ad is skipped.

Fixes #1148

[codecov-commenter]

Codecov Report

Base: 54.21% // Head: 54.13% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (2ea5cfc) compared to base (77fb07b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
- Coverage   54.21%   54.13%   -0.08%     
==========================================
  Files         148      147       -1     
  Lines       14757    14739      -18     
==========================================
- Hits         8000     7979      -21     
- Misses       5884     5887       +3     
  Partials      873      873              

Impacted Files	Coverage Δ
internal/ingest/linksystem.go	72.40% <100.00%> (+0.38%)	⬆️
dagsync/test/util.go	80.09% <0.00%> (-2.37%)	⬇️
dagsync/dtsync/sync.go	70.22% <0.00%> (-2.25%)	⬇️
fsutil/disk/usage_windows.go
internal/ingest/ingest.go	69.33% <0.00%> (+0.75%)	⬆️

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