Support classes with no public properties in toDataFrame converter

Author: nikitinas

src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/DataFrameImpl.kt

@@ -35,7 +35,7 @@ internal open class DataFrameImpl<T>(cols: List<AnyCol>, val nrow: Int) : DataFr
         // check that column sizes are equal
         val invalidSizeColumns = cols.filter { it.size != nrow }
         require(invalidSizeColumns.isEmpty()) {
-            "Unequal column sizes:\n${cols.joinToString("\n") { it.name + ": " + it.size }}"
+            "Unequal column sizes. Expected rows count = $nrow, but was:\n${cols.joinToString("\n") { it.name + ": " + it.size }}"
         }
 
         // check that column names are unique

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

@@ -204,5 +204,7 @@ internal fun convertToDataFrame(
             }
         }
     }
-    return dataFrameOf(columns)
+    return if (columns.isEmpty()) {
+        DataFrame.empty(data.count())
+    } else dataFrameOf(columns)
 }

tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/toDataFrame.kt

@@ -116,4 +116,17 @@ class CreateDataFrameTests {
             it.kind shouldBe ColumnKind.Value
         }
     }
+
+    @Test
+    fun `convert type with no properties`() {
+        class Child
+        class Entry(val a: Int, val child: Child)
+
+        val df = listOf(Entry(1, Child())).toDataFrame(depth = 100)
+        df.rowsCount() shouldBe 1
+
+        val childCol = df[Entry::child]
+        childCol.kind() shouldBe ColumnKind.Group
+        childCol.asColumnGroup().columnsCount() shouldBe 0
+    }
 }