Fix the NPE in getClientBlockInfo while INode is a DIR

info.blockIds.size() doesn't check null pointer in getClientBlockInfo. It causes NPE while trying to getClientBlockInfo for a DIR. This problem can be reproduced by running following command:
```
$> bin/tachyon tfs location /
```
Now, it will check the INode type and return back an exception like
```
FileDoesNotExistException(message: File 1 is a folder.)
```
Any comment?