The command `ls-projects -t` always fails with the error:
Fatal: --tree and --show-branch options are not compatible.
This is because when there are no --show-branch options passed, the
call to `getShowBranch` still returns an empty list, which causes
`getShowBranch() != null` to be true, and results in the error condition.
Update the condition to first check if the list is null, and then if
it's empty.
The error only needs to be emitted if the list is not null and not
empty.
Change-Id: Ia4db17adaa814328da6eb88874088bbe3b6acbe6
6a4b4d27e3