Fix aip docs null ex (#736)

* Fix possible null pointer exception in ApiDocsDubboGenericUtil#invoke

* fix api docs null exception
diff --git a/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/controller/ApiDocsController.java b/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/controller/ApiDocsController.java
index 51ac909..3941d64 100644
--- a/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/controller/ApiDocsController.java
+++ b/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/controller/ApiDocsController.java
@@ -198,7 +198,9 @@
                 "java.lang.Character".equals(typeStr) ||
                 "java.lang.Short".equals(typeStr) ||
                 "java.lang.Boolean".equals(typeStr) ||
-                "java.lang.String".equals(typeStr)) {
+                "java.lang.String".equals(typeStr) ||
+                "java.math.BigDecimal".equals(typeStr) ||
+                "java.math.BigInteger".equals(typeStr)) {
             return true;
         }
         return false;