[SPARK-14637][SQL] object expressions cleanup

## What changes were proposed in this pull request?

Simplify and clean up some object expressions:

1. simplify the logic to handle `propagateNull`
2. add `propagateNull` parameter to `Invoke`
3. simplify the unbox logic in `Invoke`
4. other minor cleanup

TODO: simplify `MapObjects`

## How was this patch tested?

existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12399 from cloud-fan/object.