Added a default value of EPOCH if migration from string to timestamp fails

[aditya-07]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2116

Description

1. Added a default value of EPOCH if migration from string to timestamp fails using COALESCE .
2. Updated Migration Test to check corrupted timestamp migrates to Instant.EPOCH .

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[jingtang10]

thanks aditya for following up on this one!