[analyzer] An ObjC for-in loop runs 0 times if the collection is nil.

In an Objective-C for-in loop "for (id element in collection) {}", the loop
will run 0 times if the collection is nil. This is because the for-in loop
is implemented using a protocol method that returns 0 when there are no
elements to iterate, and messages to nil will result in a 0 return value.

At some point we may want to actually model this message send, but for now
we may as well get the nil case correct, and avoid the false positives that
would come with this case.

<rdar://problem/13744632>