Fix bug in explode.

Author: nikitinas

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

@@ -33,6 +33,7 @@ internal fun AnyCol.explodeImpl(): AnyCol = dataFrameOf(this).explodeImpl(true)
 
 internal fun <T> DataFrame<T>.explodeImpl(dropEmpty: Boolean = true, columns: ColumnsSelector<T, *>): DataFrame<T> {
     val columns = getColumnsWithPaths(columns)
+    if (columns.isEmpty()) return this
 
     val rowExpandSizes = indices.map { row ->
         columns.maxOf {

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

@@ -28,4 +28,10 @@ class ExplodeTests {
             1, 3, 6, 7, null
         )
     }
+
+    @Test
+    fun `explode nothing`() {
+        val df = dataFrameOf("a", "b")(1, 2)
+        df.explode() shouldBe df
+    }
 }