Mark _deviceForNodeID:createIfNeeded: as possibly returning nil. (project-chip#36617)

Author: bzbarsky-apple

src/darwin/Framework/CHIP/MTRDeviceController.mm

@@ -371,7 +371,7 @@ - (MTRDevice *)_setupDeviceForNodeID:(NSNumber *)nodeID prefetchedClusterData:(N
     return [[MTRDevice alloc] initForSubclassesWithNodeID:nodeID controller:self];
 }
 
-- (MTRDevice *)_deviceForNodeID:(NSNumber *)nodeID createIfNeeded:(BOOL)createIfNeeded
+- (MTRDevice * _Nullable)_deviceForNodeID:(NSNumber *)nodeID createIfNeeded:(BOOL)createIfNeeded
 {
     std::lock_guard lock(*self.deviceMapLock);
     MTRDevice * deviceToReturn = [_nodeIDToDeviceMap objectForKey:nodeID];

src/darwin/Framework/CHIP/MTRDeviceController_Internal.h

@@ -107,7 +107,8 @@ NS_ASSUME_NONNULL_BEGIN
 #pragma mark - Device-specific data and SDK access
 // DeviceController will act as a central repository for this opaque dictionary that MTRDevice manages
 - (MTRDevice *)deviceForNodeID:(NSNumber *)nodeID;
-- (MTRDevice *)_deviceForNodeID:(NSNumber *)nodeID createIfNeeded:(BOOL)createIfNeeded;
+// _deviceForNodeID:createIfNeeded: can only return nil if NO is passed for createIfNeeded.
+- (MTRDevice * _Nullable)_deviceForNodeID:(NSNumber *)nodeID createIfNeeded:(BOOL)createIfNeeded;
 /**
  * _setupDeviceForNodeID is a hook expected to be implemented by subclasses to
  * actually allocate a device object of the right type.