resolc fails on deploy-only Yul objects

`resolc --yul` fails when a Yul object has no `_deployed` sub-object, while `solc` compiles it fine.

```yul
object "Test" {
    code {
        { mstore(0, 42) return(0, 32) }
    }
}
```

`__runtime` is created unconditionally even when no runtime code exists, leaving its entry block without a terminator.