eligible_at in mainnet_builder_block_submisison for failed simulations

[taarushv]

Describe the bug When sim_success return false, the eligible_at field is set to January 1, 0001, 12:00 AM, I assume probably because we initialize the field to null time here

Steps to reproduce Example of such existing record: select * from mainnet_builder_block_submission where sim_success=false and block_number=17322049 limit 1

Expected behavior Ideally, for cases where the simulation fails and the submission is never eligible, a null or a empty field would be more appropriate, to avoid skewing high level metrics that transform eligible_at