Only delete the target__ref if target is defined

I had a $ref that couldn't be resolved (because I had mislabeled it - see the commit that fixes it [here](https://github.com/open-contracting/standard/commit/15d11361b58d714b9fd57e63f6f17808a88ff2ee).

But the result was without this if statement nothing renders at all. With this, you just see the $ref literal instead of the embedded definition, which seems correct.

Thanks for your consideration.