Always return an array from SwiftFileBackend::addMissingMetadata

Correct the behavior of SwiftFileBackend::addMissingMetadata to match
the documentation and calling code expectations by always returning an
array.

Bug: 60477
Change-Id: I8d12888a6ebfa23502f8cf3a28866dcb00a18ec8
Merge "Always return an array from SwiftFileBackend::addMissingMetadata"