Explicit undefined values in a map should be equivalent to absence of the value

[yaacovCR]

In some areas of the code base, we treat the special JS undefined value in a map as equivalent to absence of the value in the map, but this is inconsistent.

We should be consistent across the code base.

See:

• fix(variables): treat undefined as absent with respect to variables #4708
• fix: ignore undefined-valued unknown fields in input objects #4710
• fix(OneOf): fail coercion for OneOf is the single field is invalidly provided by a default #4719