[SPARK-3063][SQL] ExistingRdd should convert Map to catalyst Map.

Currently `ExistingRdd.convertToCatalyst` doesn't convert `Map` value.

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

Closes #1963 from ueshin/issues/SPARK-3063 and squashes the following commits:

3ba41f2 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-3063
4d7bae2 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-3063
9321379 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-3063
d8a900a [Takuya UESHIN] Make ExistingRdd.convertToCatalyst be able to convert Map value.