Fix serialization for DataColumn<List?> and for unnamed columns

Author: koperagen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/json.kt

@@ -211,7 +211,7 @@ internal fun KlaxonJson.encodeRow(frame: ColumnsContainer<*>, index: Int): JsonO
             col is ColumnGroup<*> -> encodeRow(col, index)
             col is FrameColumn<*> -> col[index]?.let { encodeFrame(it) }
             col.isList() -> {
-                array(col[index] as List<*>)
+                col[index]?.let { array(it as List<*>) } ?: array()
             }
             col.typeClass in valueTypes -> {
                 val v = col[index]
@@ -229,8 +229,13 @@ internal fun KlaxonJson.encodeRow(frame: ColumnsContainer<*>, index: Int): JsonO
 internal fun KlaxonJson.encodeFrame(frame: AnyFrame): JsonArray<*> {
     val allColumns = frame.columns()
 
+    // if there is only 1 column, then `isValidValueColumn` always true.
+    // But at the same time, we shouldn't treat dataFrameOf("value")(1,2,3) like unnamed column
+    // because it was created by user.
+    val isPossibleToFindUnnamedColumns = allColumns.size != 1
     val valueColumn = allColumns.filter { it.name.startsWith(valueColumnName) }
-        .maxByOrNull { it.name }?.let { valueCol ->
+        .takeIf { isPossibleToFindUnnamedColumns }
+        ?.maxByOrNull { it.name }?.let { valueCol ->
             if (valueCol.kind() != ColumnKind.Value) null
             else {
                 // check that value in this column is not null only when other values are null
@@ -248,7 +253,8 @@ internal fun KlaxonJson.encodeFrame(frame: AnyFrame): JsonArray<*> {
         }
 
     val arrayColumn = frame.columns().filter { it.name.startsWith(arrayColumnName) }
-        .maxByOrNull { it.name }?.let { arrayCol ->
+        .takeIf { isPossibleToFindUnnamedColumns }
+        ?.maxByOrNull { it.name }?.let { arrayCol ->
             if (arrayCol.kind() == ColumnKind.Group) null
             else {
                 // check that value in this column is not null only when other values are null

tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/json.kt

@@ -1,6 +1,7 @@
 package org.jetbrains.kotlinx.dataframe.io
 
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldContain
 import org.jetbrains.kotlinx.dataframe.DataFrame
 import org.jetbrains.kotlinx.dataframe.api.convert
 import org.jetbrains.kotlinx.dataframe.api.dataFrameOf
@@ -139,6 +140,23 @@ class JsonTests {
         DataFrame.readJsonStr(df.toJson()) shouldBe df
     }
 
+    @Test
+    fun `list serialization with nulls`() {
+        val df = dataFrameOf("a")(listOf(1, 2, 3), null)
+        val text = df.toJson()
+        val df1 = DataFrame.readJsonStr(text)
+        df1["a"][1] shouldBe emptyList<Int>()
+    }
+
+    @Test
+    fun `serialize column with name 'value'`() {
+        val df = dataFrameOf("a")(dataFrameOf("value")(1, 2, 3))
+        val json = df.toJson()
+        json shouldContain "\"value\":1"
+        val df1 = DataFrame.readJsonStr(json)
+        df shouldBe df1
+    }
+
     @Test
     fun `literal json field named 'value'`() {
         val json = """