[SPARK-3429] Don't include the empty string "" as a defaultAclUser

Changes logging from

```
14/09/05 02:01:08 INFO SecurityManager: Changing view acls to: aash,
14/09/05 02:01:08 INFO SecurityManager: Changing modify acls to: aash,
14/09/05 02:01:08 INFO SecurityManager: SecurityManager: authentication disabled; ui acls disabled; users with view permissions: Set(aash, ); users with modify permissions: Set(aash, )
```
to
```
14/09/05 02:28:28 INFO SecurityManager: Changing view acls to: aash
14/09/05 02:28:28 INFO SecurityManager: Changing modify acls to: aash
14/09/05 02:28:28 INFO SecurityManager: SecurityManager: authentication disabled; ui acls disabled; users with view permissions: Set(aash); users with modify permissions: Set(aash)
```

Note that the first set of logs have a Set of size 2 containing "aash" and the empty string ""

cc tgravescs

Author: Andrew Ash <andrew@andrewash.com>

Closes #2286 from ash211/empty-default-acl and squashes the following commits:

18cc612 [Andrew Ash] Use .isEmpty instead of ==""
cf973a1 [Andrew Ash] Don't include the empty string "" as a defaultAclUser