Nullable Numerics fail in `validate-data`

[vjeeva]

Problem:

• Tables with nullable numeric data types are breaking the validate-data function.
• If a row has a NaN in Postgres, AsyncPG reads that as Python Object Decimal('NaN').
• Python also thinks that Decimal('NaN') == Decimal('NaN') -> False.

Solution

• Reference: https://magicstack.github.io/asyncpg/current/usage.html#example-decoding-numeric-columns-as-floats
• Basically, encode and decode numeric types as strings to make things consistent.