PyObject doesn't work with None default

[samuelcolvin]

Bug

import pydantic

class Foo(pydantic.BaseModel):
    x: pydantic.PyObject = None
    

Foo()

~/code/pydantic/pydantic/utils.py in import_string(dotted_path)
    102     """
    103     try:
--> 104         module_path, class_name = dotted_path.strip(' ').rsplit('.', 1)
    105     except ValueError as e:
    106         raise ImportError(f'"{dotted_path}" doesn\'t look like a module path') from e

AttributeError: 'NoneType' object has no attribute 'strip'