Update djpyfs.py

Sometimes the attribute exists but is empty.
[Here are the docs for `getattr`](https://docs.python.org/2/library/functions.html#getattr). Basically, `getattr` works like `hasattr`, but returns the attribute if it exists. If the attribute doesn't exist, it will return the third argument. `if` will simply check the truthiness/falsiness of whatever `getattr` returns. Since I've set the third argument to `False`, we'll hit the `else` statement if `settings.DJFS` doesn't exist, but we'll also hit the `else` statement if `settings.DJFS` exists and is a falsy value (like `None`).