Update RCTLinkingManager.h to explicitly state the 'nullability' of parameters (#20798)

Summary:
Fixes #20797

As mentioned in #20797 when running `react-native run-ios Xcode 9.2 will complain about the nullability of pointers in `RCTLinkingManager.h`.
Pull Request resolved: #20798

Differential Revision: D9988581

Pulled By: hramos

fbshipit-source-id: e3ce7736da97d314a421c2c1ab71577864081642

Author: Warren Knox

Libraries/LinkingIOS/RCTLinkingManager.h

@@ -11,17 +11,17 @@
 
 @interface RCTLinkingManager : RCTEventEmitter
 
-+ (BOOL)application:(UIApplication *)app
-            openURL:(NSURL *)URL
-            options:(NSDictionary<UIApplicationOpenURLOptionsKey,id> *)options;
++ (BOOL)application:(nonnull UIApplication *)app
+            openURL:(nonnull NSURL *)URL
+            options:(nonnull NSDictionary<UIApplicationOpenURLOptionsKey, id> *)options;
 
-+ (BOOL)application:(UIApplication *)application
-            openURL:(NSURL *)URL
-  sourceApplication:(NSString *)sourceApplication
-         annotation:(id)annotation;
++ (BOOL)application:(nonnull UIApplication *)application
+              openURL:(nonnull NSURL *)URL
+    sourceApplication:(nullable NSString *)sourceApplication
+           annotation:(nonnull id)annotation;
 
-+ (BOOL)application:(UIApplication *)application
-continueUserActivity:(NSUserActivity *)userActivity
-  restorationHandler:(void (^)(NSArray * __nullable))restorationHandler;
++ (BOOL)application:(nonnull UIApplication *)application
+    continueUserActivity:(nonnull NSUserActivity *)userActivity
+      restorationHandler:(nonnull void (^)(NSArray *__nullable))restorationHandler;
 
 @end