[fix] bootstrap balances should be done only for genesis block testing

[shrimalmadhur]

Fixes # .

Motivation

When we run parallel validation from any block other then genesis, we are using bootstrap_balances.json to initialize balances which results into balance mismatch because actual balances in n-1 block might have been updated and we need to use that.

Solution

Add a condition to validation that, bootstrap balances only if startIndex is empty or is genesis block.

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