Handle `null` being provided to `type` when we do an optimized `anyOf` check

[mwildehahn]

We try and optimize anyOf checks for types that all have a specific string field that acts like an enum. This optimized check isn't handling if that provided value is given as null here:

hack-json-schema/src/Codegen/Constraints/UntypedBuilder.php

Line 433 in 04b77ea

->addAssignment('$type_name', '($typed[$key] ?? null) as nonnull', HackBuilderValues::literal())

.