Add ?? with nullsafe or default value.

refs: https://github.com/google/jsonnet/issues/930

better to have it as 
```
maybeNullObj?['someField'] ?? 'defaultValue'
```

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing
refs in sjsonnet: https://github.com/databricks/sjsonnet/issues/328