documentation recommends accessing `config.cache` in an unsafe way

[DetachHead]

https://docs.pytest.org/en/stable/how-to/cache.html#the-new-config-cache-object

in this example recommends accessing pytestconfig.cache which will crash at runtime if the cacheprovider plugin has been disabled as mentioned here:

_______________________________________________________________________________ ERROR at setup of test_function ________________________________________________________________________________

pytestconfig = <_pytest.config.Config object at 0x749189eaa7b0>

    @pytest.fixture
    def mydata(pytestconfig):
>       val = pytestconfig.cache.get("example/value", None)
E       AttributeError: 'Config' object has no attribute 'cache'

python/tests/test_asdf.py:12: AttributeError
=================================================================================== short test summary info ====================================================================================
ERROR python/tests/test_asdf.py::test_function - AttributeError: 'Config' object has no attribute 'cache'
======================================================================================= 1 error in 0.07s =======================================================================================

pytest should set the cache object to None when the plugin isn't enabled. that way, type checkers will report an error when attempting to use it without checking whether it's None

[RonnyPfannschmidt]

The plugin itself creates the cache attribute

When intentionally disabling the cache id rather replace it with a fitting null object

This could be part of having a cache policy in general as people also want a xdg based storage