Handle empty EarliestCommitToRetain

[alexr17]

Important Read

• Please ensure the GitHub issue is mentioned at the beginning of the PR

What is the purpose of the pull request

If the earliest commit to retain is empty (rare occurrences):

• Get all instants after lastSyncedInstant.
• If no clean instants since lastSyncedInstant, we do incremental sync.
• Otherwise if earliestCommitToRetain.isEmpty() and there are clean instants, do a full snapshot sync to err on the side of caution.

Brief change log

Handles case when earliest commit to retain is empty or null

Verify this pull request

This change added tests and can be verified as follows:

• Added unit tests