Fix listxattr() response when name list is empty
FUSE_LISTXATTR returns a xattr name list. It is possible that all xattrs have been filtered out by server and resulting name list is empty. As of now rust version of code writes a null byte to client supplied buffer. This fails the kernel fuse code verification and return -EIO to user space.
Fix this by not writing a single null byte. Simply send a response back with error code 0 and virtiofs kernel code should be able to handle it and update variable argument size to 0.