Always replicate vectors metadata

[javitonino]

Vectors metadata was not being replicated when no segments changed. This is wrong because of two things:

• Deletions are only kept in metadata, so deleting a resource wouldn't be replicated.
• Empty indexes / empty KBs won't replicate anything and fail to open

The second case used to work before #2492 because we created a dummy shard which included some empty metadata files for the vectors index.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.19%. Comparing base (4b324d2) to head (cf2cf24).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2495      +/-   ##
==========================================
- Coverage   86.23%   86.19%   -0.05%     
==========================================
  Files         375      375              
  Lines       23395    23395              
==========================================
- Hits        20175    20165      -10     
- Misses       3220     3230      +10     

Flag	Coverage Δ
nucliadb	73.97% <ø> (-0.02%)	⬇️
nucliadb-ingest	29.08% <ø> (ø)
nucliadb-reader	25.01% <ø> (ø)
nucliadb-search	37.24% <ø> (ø)
nucliadb-standalone	47.00% <ø> (ø)
nucliadb-train	45.62% <ø> (ø)
nucliadb-writer	39.96% <ø> (ø)
nucliadb_dataset	55.31% <ø> (ø)
nucliadb_models	85.08% <ø> (ø)
nucliadb_sdk	80.11% <ø> (ø)
nucliadb_sidecar	87.24% <ø> (-1.79%)	⬇️
nucliadb_telemetry	86.55% <ø> (ø)
nucliadb_utils	84.01% <ø> (ø)

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