ignore_exc should return default values

[Justylol]

Its better for ignore_exc to use default value on return, instead of None. It will be real missing cache behaviour and will help with errors when default expected. For example:

Django==3.2
djangorestframework==3.11.1
pymemcache==3.5.0

rest_framework.throttling module

        self.history = self.cache.get(self.key, [])
        self.now = self.timer()

        # Drop any requests from the history which have now passed the
        # throttle duration
        while self.history and self.history[-1] <= self.now - self.duration:
            self.history.pop()
        if len(self.history) >= self.num_requests:
            return self.throttle_failure()

This code with in Django with PyMemcacheCache cache backend and ignore_exc option leads to an error object of type 'NoneType' has no len() because instead of returning list as missing cache result, it returns None.

[jogo]

Thank you for the bug report, #351 should address this