Fix issue with `anyOf` containing a single value

[mwildehahn]

If anyOf references a single schema, there is an error when using unique references here:

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

Line 358 in 7d946d3

$this->current_type = $schema_builders[0]->getType();

. We require the schema to be built before accessing the type.