Simplify SubscriptionEventStreamBatch type

[mtesseract]

Currently SubscriptionEventStreamBatch is defined as follows

data SubscriptionEventStreamBatch a = SubscriptionEventStreamBatch
  { _cursor :: SubscriptionCursor -- ^ cursor for this subscription batch
  , _events :: Maybe (Vector a) -- ^ Events for this subscription batch
  } deriving (Show, Generic)

In particular, _events is defined as a Maybe (Vector a). As I see it, we don't need the extra Maybe wrapping at this point. The case "no events field present in Nakadi batch" could be handled just like "empty events array in Nakadi batch".