fix(iota-indexer): multi_fetch early return if input is empty

[sergiupopescu199]

Description of change

This PR fixes an edge case where the multi_fetch entered in deadlock if the input was an empty vector.

Links to any relevant issues

fixes #9508

How the change has been tested

Providing an empty vector as input no longer halts the whole execution.

• Basic tests (linting, compilation, formatting, unit/integration tests)
• Patch-specific tests (correctness, functionality coverage)

Infrastructure QA (only required for crates that are maintained by @iotaledger/infrastructure)

• Synchronization of the indexer from genesis for a network including migration objects.
• Restart of indexer synchronization locally without resetting the database.
• Restart of indexer synchronization on a production-like database.
• Deployment of services using Docker.
• Verification of API backward compatibility.

Note

This patch does not affect the normal operation of the indexer, thus no other test were conducted.

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6 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
apps-backend	Ignored			Dec 4, 2025 9:02am
apps-ui-kit	Ignored			Dec 4, 2025 9:02am
iota-evm-bridge	Ignored			Dec 4, 2025 9:02am
iota-multisig-toolkit	Ignored			Dec 4, 2025 9:02am
rebased-explorer	Ignored			Dec 4, 2025 9:02am
wallet-dashboard	Ignored			Dec 4, 2025 9:02am