$create parameter in assign::get_user_submission() not optional as documented

The PHPDoc for the $create parameter in assign::get_user_submission() is

@param bool $create optional - defaults to false. If set to true a new submission object
will be created in the database with the status set to "new".

This is not strictly correct, as the parameter has no default. If called without the parameter, the method happens to behave as if false was passed, but a warning message is emitted. It would be better if it were actually to default to false as documented.