builtins PosixCommands ls command minor formatting glitch

[paulk-asert]

Running org.jline.demo.examples.PosixCommandsRegistryExample gives:

posix> ls
            ..          sample1.txt sample2.txt

The first entry is normally expected to be "." but it is "".

[paulk-asert]

With the proposed PR, the output becomes:

posix> ls
.           ..          sample1.txt sample2.txt