Parsing: Negate the condition when `if` branch is empty.

I.e. parse

```cabal
if flag(some-condition)
buildable: True
else
buildable: False
```

as

```cabal
if !flag(some-condition)
buildable: False
```

And / Or: issue warning if this is the case, so `.cabal` file can be edited by author.

---

Motivation: is that the above would be pretty printed as

```cabal
if flag(some-condition)
else
buildable: False
```

which is ugly.