fixup! introducing parsing of Char? columns. It works the same as String parsing, failing when the returned type is Char or String

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/parse.kt

@@ -730,6 +730,10 @@ internal fun <T> DataFrame<T>.parseImpl(options: ParserOptions?, columns: Column
                     .asColumnGroup(col.name())
                     .asDataColumn()
 
+            // Base case, parse the column if it's a `Char?` column
+            col.isSubtypeOf<Char?>() ->
+                col.map { it?.toString() }.tryParseImpl(options)
+
             // Base case, parse the column if it's a `String?` column
             col.isSubtypeOf<String?>() ->
                 col.cast<String?>().tryParseImpl(options)