[Mac] Fix accessibility crash because I am terrible with dynamic types.

Turns out attributes like AXPosition will crash with @[] :/

BUG=412707
TEST=Start Chrome with --enable-new-avatar-menu. Provided you have zooming
enabled in your Mac Accessibility settings, press Ctrl and scroll up to zoom
in. Click on the avatar button. The avatar bubble should open and Chrome should
not crash.

Review URL: https://codereview.chromium.org/645933002

Cr-Commit-Position: refs/heads/master@{#299160}

chrome/browser/ui/cocoa/profiles/profile_chooser_controller.mm

@@ -780,7 +780,7 @@ class ActiveProfileObserverBridge : public AvatarMenuObserver,
 }
 
 - (id)accessibilityAttributeValue:(NSString*)attribute {
-  return @[];
+  return nil;
 }
 
 - (BOOL)canBecomeKeyView {