casting to map data type

1. `cast (null as map )` -> `NULL of type map `
2. `cast ( map () as map )` -> `{} of type map `
3. `cast ( map (key1, val1, ...) as map )` -> `map (cast (key1 as KEY), cast (val1 as VALUE), ...)`, as long as `typeof (key1) convertible to KEY, typeof (val1) convertible to VALUE`

`KEY` and `VALUE` can be any valid type

Look for map casting logic in the SQL type system and expression evaluation code. The issue describes three specific casting behaviors for map types. Find existing cast implementations for other types to understand the pattern. Test with map constructors and type conversions to verify the new behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.