Allow TransitionWithPresentation to return nil for conditional presen…

…tation.

Summary: Returning nil means no presentation controller will be used.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, randcode-generator

Reviewed By: randcode-generator

Tags: #material_motion

Differential Revision: http://codereview.cc/D3229

src/transitions/Transition.swift

@@ -70,10 +70,12 @@ public protocol TransitionWithPresentation: Transition {
 
    The returned presentation controller may choose to conform to WillBeginTransition in order to
    associate reactive motion with the transition.
+
+   If nil is returned then no presentation controller will be used.
    */
   func presentationController(forPresented presented: UIViewController,
                               presenting: UIViewController?,
-                              source: UIViewController) -> UIPresentationController
+                              source: UIViewController) -> UIPresentationController?
 }
 
 /**