Commit c2d7461d3cac639778e7740c4b08e459332a6b14 dropped a usage of
`last_valuation_rate` around this code block. After that, it was always checked
although the value would be None as it was being explicitly set above.
Signed-off-by: Akhil Narang <me@akhilnarang.dev>
dd911aa521