[chore]: proveUnknownValidatorsToPDG must validate _witnesses length

[tamtamchik]

Summary

Instead of reverting here when _witnesses.length is 0 nothing happens:

for (uint256 i = 0; i < _witnesses.length; i++) {
    VAULT_HUB.proveUnknownValidatorToPDG(address(_stakingVault()), _witnesses[i]);
}

Expected Behavior

Transaction reverts

Potential Impact

The transaction can be mistakengly counted as successful.

Steps to Reproduce

https://hoodi.etherscan.io/tx/0x1ce0934c79c33dc8c6bdc387fda6f56de094ab0f2e5b8e6707fd0fd1a7a0d7f5

Possible Solutions

No response

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