build: 优化依赖

core-bukkit/src/main/java/cn/hamster3/mc/plugin/core/bukkit/api/CoreBukkitAPI.java

@@ -99,22 +99,22 @@ public final class CoreBukkitAPI extends CoreAPI {
     }
 
     @Override
-    public Gson getGson() {
+    public @NotNull Gson getGson() {
         return gson;
     }
 
     @Override
-    public Gson getHumanGson() {
+    public @NotNull Gson getHumanGson() {
         return humanGson;
     }
 
     @Override
-    public ExecutorService getExecutorService() {
+    public @NotNull ExecutorService getExecutorService() {
         return executorService;
     }
 
     @Override
-    public ScheduledExecutorService getScheduledExecutorService() {
+    public @NotNull ScheduledExecutorService getScheduledExecutorService() {
         return scheduledExecutorService;
     }
 }