The _native computed property that backs this method has a different
contract than requestNativeBuffer() expected: it returned an empty
buffer rather than using an optional and returning nil. We were just
wrapping up that empty buffer and declaring success, which meant we
would bridge from an NSArray wrapping native storage of T to an empty
native array, always.
This change is a necessary prerequisite to importing NSArray* as
(AnyObject[])! per <rdar://problem/16535097>. That (imminent) change
tests this, but we need more targetted testing of this area.
Swift SVN r18143
