Use Preconditions.checkNotNull in FirebaseFunctions. (#475)

Author: rlazo

firebase-functions/src/main/java/com/google/firebase/functions/FirebaseFunctions.java

@@ -194,9 +194,7 @@ URL getURL(String function) {
    * @param origin The origin of the local emulator, such as "http://10.0.2.2:5005".
    */
   public void useFunctionsEmulator(@NonNull String origin) {
-    if (origin == null) {
-      throw new IllegalArgumentException("origin cannot be null");
-    }
+    Preconditions.checkNotNull(origin, "origin cannot be null");
     urlFormat = origin + "/%2$s/%1$s/%3$s";
   }
 
@@ -230,10 +228,11 @@ Task<HttpsCallableResult> call(String name, @Nullable Object data, HttpsCallOpti
    * @return A Task that will be completed when the request is complete.
    */
   private Task<HttpsCallableResult> call(
-      String name, @Nullable Object data, HttpsCallableContext context, HttpsCallOptions options) {
-    if (name == null) {
-      throw new IllegalArgumentException("name cannot be null");
-    }
+      @NonNull String name,
+      @Nullable Object data,
+      HttpsCallableContext context,
+      HttpsCallOptions options) {
+    Preconditions.checkNotNull(name, "name cannot be null");
     URL url = getURL(name);
 
     Map<String, Object> body = new HashMap<>();