Make response member in OkHttpMethodBase nullable

The class uses non-null lateinit variable that
is not initialized. The member should be of
nullable type.

Fixes nextcloud/android#5073
Fixes nextcloud/android#5102
Fixes nextcloud/android#5100
Fixes nextcloud/android#5142
Fixes nextcloud/android#5156

Signed-off-by: Chris Narkiewicz <hello@ezaquarii.com>