Libsass incorrectly allows the concatenation of `null`

Libsass allows the following where Ruby sass errors with 

> Invalid null operation: ""foo" plus null".

``` scss
a {
  a: foo + null;
}
```

http://sassmeister.com/gist/4e3c06cbdb252c553132

Originally mentioned by @jakob-e in https://github.com/sass/libsass/issues/674#issuecomment-64180089
