Make ol.Map#setView do not accept undefined

With this change `map.setView(null)` is permitted, but `map.setView(undefined)` (or `map.setView()`) is not. `map.getView` now always returns a defined value, which may be `null` is `setView` was called with `null`.

Also, this patch fixes a bug where a `TypeError` occurred when `map.setView(null)` was called. The error occurred because the `renderFrame_` function did not consider the case where `getView()` returned `null`.

Please review.
oops yes, good catch!