support "ls /"

[paulk-asert]

When running ./mvx demo PosixCommandsRegistryExample and invoking the ls / command, the following error occurs (at least on macOS):

Error: Cannot invoke "java.nio.file.Path.toString()" because the return value of "java.nio.file.Path.getFileName()" is null

The JavaDoc for Path#getFileName says it can return null. With this PR, the command works as expected, showing the files in the root directory.