fix empty and nonEmpty matchers

With **master** branch:

```
> ass('DrSlump').string.nonEmpty.test();
AssError:

 Passed: to be a string
 Failed: is not empty
    But: was DrSlump

    at repl:1:22
    at REPLServer.self.eval (repl.js:110:21)
    at repl.js:249:20
    at REPLServer.self.eval (repl.js:122:7)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)
```

With **feature/fix-non-empty-empty-matchers** branch:

> ass('DrSlump').string.nonEmpty.test();
true


With **master** branch:

```
> ass('').string.empty.test();
AssError:

 Passed: to be a string
 Failed: is empty
    But: was

    at repl:1:15
    at REPLServer.self.eval (repl.js:110:21)
    at repl.js:249:20
    at REPLServer.self.eval (repl.js:122:7)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)
```

With **feature/fix-non-empty-empty-matchers** branch:

> ass('').string.empty.test();
true