[BUG] When task status statistics and process status statistics are empty, pie charts are not displayed on the homepage and project homepage


![image](https://user-images.githubusercontent.com/55787491/78202013-b60e6f00-74c5-11ea-8632-a6b5f8d03fa2.png)
![image](https://user-images.githubusercontent.com/55787491/78202027-c0c90400-74c5-11ea-89b5-099e85775d72.png)



**Which version of Dolphin Scheduler:**
 -[refactor-worker , dev]

