[SPARK-18398][SQL] Fix nullabilities of MapObjects and ExternalMapToCatalyst.

## What changes were proposed in this pull request?

The nullabilities of `MapObject` can be made more strict by relying on `inputObject.nullable` and `lambdaFunction.nullable`.

Also `ExternalMapToCatalyst.dataType` can be made more strict by relying on `valueConverter.nullable`.

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #15840 from ueshin/issues/SPARK-18398.