fix: add missing nullness annotation

Author: sonallux

spotify-web-api-java/src/main/java/de/sonallux/spotify/api/http/ApiCall.java

@@ -78,7 +78,7 @@ public Response<T> executeCall() throws IOException {
         return parseResponse(call.execute());
     }
 
-    public T execute() throws SpotifyApiException {
+    public @Nullable T execute() throws SpotifyApiException {
         Response<T> response;
         try {
             response = executeCall();