update_fields: [] performs full document reindex instead of no-op

[dimitrikochnev]

When passing an empty array to update_fields, Chewy performs a full document reindex instead of treating it as a no-op (no fields to update). This is unexpected and dangerous behavior.

Expected Behavior

update_fields: [] should mean "update zero fields" - i.e., do nothing or skip the update. An empty array is an explicit empty set.

If "update all fields" is intended, it should be triggered by update_fields: nil or by omitting the option entirely.

Actual Behavior

update_fields: [] triggers a full document replacement, overwriting all fields with current model values.

Why This Is Problematic

Consider a pipeline that updates different transient (so I don't store fields in AR, they re simple attribute accessors) field groups:

DocumentIndex.import(
  [document],  
  direct_import: true,  
  update_fields: document.class.transient_attributes(fields_group) # --> []
)
end

This is a silent data loss bug that's very hard to debug.