Massa event cache key ident estimation can be optimized

[Leo-Besancon]

In case we query events with a filter with slot_start that is after the slot of the last event in the db, the slots_since function returns an Err() variant and we have the following WARN:

2025-01-10T13:56:05.906052Z  WARN massa_event_cache::event_cache: Could not estimate count for key indent: Event - filter_item: SlotStart(Slot { period: 2014222, thread: 15 }): period overflow error

In that case, we also return max_entry_count as the estimate.

In that specific case, we want the estimation to return 0 instead (because we have no events in the given period) to avoid unnecessary work.