Explicit representation of star in MergeIntoTable's Update and Insert action

Currently, UpdateAction and InsertAction in the MergeIntoTable implicitly represent `update set *` and `insert *` with empty assignments. That means there is no way to differentiate between the representations of "update all columns" and "update no columns". For SQL MERGE queries, this inability does not matter because the SQL MERGE grammar that generated the MergeIntoTable plan does not allow "update no columns". However, other ways of generating the MergeIntoTable plan may not have that limitation, and may want to allow specifying "update no columns". For example, in the Delta Lake project we provide a type-safe Scala API for Merge, where it is perfectly valid to produce a Merge query with an update clause but no update assignments. Currently, we cannot use MergeIntoTable to represent this plan, thus complicating the generation, and resolution of merge query from scala API.

This should be fixed by having an explicit representation of * in the UpdateAction and InsertAction.