yapf adds newline to end of file when reading source file from stdin

As per #195, yapf shouldn't add a trailing newline to an empty file. It mostly doesn't, but if you run against stdin instead of a named file for some reason it does:

```
$ echo -n | yapf --style=pep8 | wc -c
1
$ touch empty
$ yapf --style=pep8 empty | wc -c
0
```

I tested this running against a2f034296536867f79e3843fb8cc3d5a90c941db under Python 3.5.1