OvershadowableManager.isComplete() looks buggy

The current implementation of `OvershadowableManager.isComplete()`:
https://github.com/apache/incubator-druid/blob/a95e3d438e676310522a68c74a43d99f5f0492d5/core/src/main/java/org/apache/druid/timeline/partition/OvershadowableManager.java#L891-L894

looks buggy because it might throw an exception when any value in `visibleGroup` is a map with more than one element. If there are never more than one element in this map, why this is a map, not just a single value?

It might be this this is not a bug because there are some invisible contract between callers to `OvershadowableManager.isComplete()` and `OvershadowableManager` which guarantee that when `isComplete()` is called all values in `visibleGroup` are maps of at most one value. But such a contract would need to be expressed explicitly in comments, at least. (Not to say that it is very fragile design and should preferably be avoided).

FYI @jihoonson

Start in core/src/main/java/org/apache/druid/timeline/partition/OvershadowableManager.java at isComplete() around lines 891-894, then inspect its callers and how visibleGroup is populated. Determine whether values with multiple map elements are valid and whether an explicit invariant exists. Done means resolving the possible exception with a documented contract or an implementation change, supported by relevant tests.

Written by the indexing model from the issue text.