option 1 - encrypt only the value if key is an empty string

this first option is simple and only modifies one line in your code to change the behavior if key is an empty string

```
$ ## example usage with empty key string
$ travis-encrypt -r kaizhu256/utility2 -k "" -v "heroku_api_key_secret"
# undefined
+UOJknUje0te1+54BFtDQi+1jFBTDfFI7qYSV+tcDMkCeQJ7K/lh4y45c8w+I/OE0BL7eyL1MowiMf5oQnSlDg==
```