Empty extra fields in config file breaks cabal

I am using `cabal user-config` to script `cabal` config changes and have noticed that removing entries results sometimes In an empty field with just a whitespace. e.g.

```
extra-library-path:
```

This breaks all compilation as cabal will now output `-L ` with a space and no path. Causing the rest of the toolchain to assume whatever is next is a path or library!

This is a bit of a showstopper for me at the moment.