issue #95 获取关注者列表userList方法，当没有关注者时，报异常

Author: chanjarster

weixin-java-mp/src/main/java/me/chanjar/weixin/mp/util/json/WxUserListGsonAdapter.java

@@ -22,9 +22,11 @@ public WxMpUserList deserialize(JsonElement json, Type typeOfT, JsonDeserializat
     wxMpUserList.setTotal(GsonHelper.getInteger(o, "total"));
     wxMpUserList.setCount(GsonHelper.getInteger(o, "count"));
     wxMpUserList.setNextOpenId(GsonHelper.getString(o, "next_openid"));
-    JsonArray data = o.get("data").getAsJsonObject().get("openid").getAsJsonArray();
-    for (int i = 0; i < data.size(); i++) {
-      wxMpUserList.getOpenIds().add(GsonHelper.getAsString(data.get(i)));
+    if (!o.get("data").isJsonNull() && !o.get("data").getAsJsonObject().get("openid").isJsonNull()) {
+      JsonArray data = o.get("data").getAsJsonObject().get("openid").getAsJsonArray();
+      for (int i = 0; i < data.size(); i++) {
+        wxMpUserList.getOpenIds().add(GsonHelper.getAsString(data.get(i)));
+      }
     }
     return wxMpUserList;
   }