Simplify logic in partner_earned_at_least_390 next node
Simplify logic in partner_earned_at_least_390 next node

For the pay leave for parents flow. I'm guessing the slightly verbose
style used here is an artefact of translating something to Ruby code
~5 years ago [1]. Given that I'm trying to read the code now, it's
worth simplifying it.

1: b6cdb9e4463fcd89b3cfa32adcdb60fec6271d64

To be clear, this commit shouldn't change the logic, just how it's
expressed in the code.

alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers