fix: add COALESCE fallback to near_amount for older delegators

[atomauro]

Enhances the PR #126 addressing transactions that only emit ft_mint and no ft_burn event return a NULL near_amount, resulting in 0 VP.

This uses a COALESCE() block:

1. It first attempts to grab the exact delta from ft_burn (This fixes the proxy inflation bug).
2. If ft_burn doesn't exist in the receipt logs, it safely falls back to extracting the amount from the ft_mint event whose owner_id matches the final receiver.