null strings fix

Author: Kopilov

dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/ArrowWriter.kt

@@ -184,8 +184,8 @@ public class ArrowWriter(
     private fun convertColumnToTarget(column: AnyCol?, targetFieldType: ArrowType): AnyCol? {
         if (column == null) return null
         return when (targetFieldType) {
-            ArrowType.Utf8() -> column.map { it.toString() }
-            ArrowType.LargeUtf8() -> column.map { it.toString() }
+            ArrowType.Utf8() -> column.map { it?.toString() }
+            ArrowType.LargeUtf8() -> column.map { it?.toString() }
             ArrowType.Binary(), ArrowType.LargeBinary() -> TODO("Saving var binary is currently not implemented")
             ArrowType.Bool() -> column.convertToBoolean()
             ArrowType.Int(8, true) -> column.convertToByte()

tests/build.gradle.kts

@@ -24,6 +24,7 @@ dependencies {
     }
     testImplementation(libs.kotlin.datetimeJvm)
     testImplementation(libs.poi)
+    testImplementation(libs.arrow.vector)
 }
 
 kotlin.sourceSets {