fix 启动空指针异常

src/main/java/com/github/yulichang/mapper/MPJTableMapperHelper.java

@@ -14,8 +14,10 @@ public class MPJTableMapperHelper {
     private static final Map<Class<?>, Class<?>> CACHE = new ConcurrentHashMap<>();
 
     public static void init(Class<?> clazz, Class<?> mapper) {
+        if (clazz != null) {
             CACHE.put(clazz, mapper);
         }
+    }
 
     public static Class<?> get(Class<?> clazz) {
         return CACHE.get(clazz);