fixed valueof method to handle null value

When I executed the following command, I got NullPointerException.
```
./bin/voldemort-admin-tool.sh --restore --url tcp://localhost:6666 --node 0
```
```
Exception in thread "main" java.lang.NullPointerException
 at voldemort.VoldemortAdminTool.main(VoldemortAdminTool.java:252)
```
So I fixed CmdUtils#valueof methods. I just added null check logic.