Ensures duplicates_lt_hash is Some if accounts lt hash is enabled

[brooksprumo]

Problem

At startup, if we're using accounts lt hash, then generate_index() is tasked with finding the duplicate accounts and tracking them. This value is used later when verifying accounts.

However, if there are no duplicates, the value (since it is an Option), is returned as None. And then verify_accounts sees the None and concludes "since this value is None, I should not do lattice-based accounts verification, and instead use the original merkle-based accounts verification".

This is wrong. Even if there are no duplicates, generate_index() must always return Some when accounts lt hash is enabled.

Summary of Changes

Ensure duplicates is always Some when accounts lt hash is enabled.

[jeffwashington]

lgtm

[HaoranYi]

lgtm.