Levkov Miron, 394, Shell

Перед nextLine() надо проверять hasNextLine(), иначе:
```
$ echo "" | java ru.fizteh.fivt.students.LevkovMiron.shell.Main 
$
: Unknown command
$
Exception in thread "main" java.util.NoSuchElementException: No line found
        at java.util.Scanner.nextLine(Scanner.java:1585)
        at ru.fizteh.fivt.students.LevkovMiron.shell.StreamShell.readCommands(StreamShell.java:12)
        at ru.fizteh.fivt.students.LevkovMiron.shell.StreamShell.readCommands(StreamShell.java:17)
        at ru.fizteh.fivt.students.LevkovMiron.shell.Main.main(Main.java:13)
```