Bug: parameter doesn't preserve default None value after yield

[natalevichmv]

When you have a task with a parameter that defaults to None, it converts to str("None") if the task is yielded from run(), but remains None if a task is returned from requires() method.

When I run the following code with luigi --module <module> YieldTask --local-scheduler, the file yield.txt contains <class 'str'>.
When I run the following code with luigi --module <module> ReturnTask --local-scheduler, the file return.txt contains <class 'NoneType'>.

import luigi

class ParamTask(luigi.Task):
    none_param = luigi.Parameter(default=None)
    name_param = luigi.Parameter()

    def run(self):
        with self.output().open('w') as f:
            f.write(str(type(self.none_param)))

    def output(self):
        return luigi.LocalTarget(f'{self.name_param}.txt')

class YieldTask(luigi.Task):
    def run(self):
        yield ParamTask(name_param='yield')

class ReturnTask(luigi.Task):
    def requires(self):
        return ParamTask(name_param='return')

luigi version: 3.2.0