Codegen for `query_required_single()`

[jrycw]

Currently, it appears that the code generation process will not generate a query using query_required_single().

After examining the _generate() function situated at edgedb-python/edgedb/codegen/generator.py, it appears there might be an if-else statement that determines the method to use: https://github.com/edgedb/edgedb-python/blob/ca401141128dba2da443657edd61e45551e53b78/edgedb/codegen/generator.py#L390-L398

Could insights be provided into the rationale behind this decision?