Fix/union all empty list

scio-core/src/main/scala/com/spotify/scio/values/SCollection.scala

@@ -65,9 +65,22 @@ object SCollection {
    * iterable is empty. For a version that accepts empty iterables, see [[ScioContext#unionAll]].
    */
   // `T: Coder` context bound is required since `scs` might be empty.
-  def unionAll[T: Coder](scs: Iterable[SCollection[T]]): SCollection[T] =
-    scs.head.context.unionAll(scs)
-
+  def unionAll[T: Coder](scs: Iterable[SCollection[T]]): SCollection[T] = {
+  scs.toList match {
+    case Nil => 
+      throw new IllegalArgumentException(
+        "SCollection.unionAll called with empty collection. Use ScioContext#unionAll for empty-safe operations."
+      )
+    case head :: Nil => head
+    case head :: tail => 
+      val tfName = head.context.tfName
+      head.context.wrap(
+        PCollectionList
+          .of((head :: tail).map(_.internal).asJava)
+          .apply(tfName, Flatten.pCollections())
+      )
+  }
+}
   /** Implicit conversion from SCollection to DoubleSCollectionFunctions. */
   implicit def makeDoubleSCollectionFunctions(s: SCollection[Double]): DoubleSCollectionFunctions =
     new DoubleSCollectionFunctions(s)

scio-core/src/main/scala/com/spotify/scio/values/SCollectionTest.scala

@@ -0,0 +1,41 @@
+"unionAll companion object" should "throw descriptive error on empty collection" in {
+  val empty = Seq.empty[SCollection[Int]]
+  val exception = the[IllegalArgumentException] thrownBy {
+    SCollection.unionAll(empty)
+  }
+  exception.getMessage should include("ScioContext#unionAll")
+}
+
+"unionAll companion object" should "handle single collection" in {
+  runWithContext { sc =>
+    val single = sc.parallelize(Seq(1, 2, 3))
+    val result = SCollection.unionAll(Seq(single))
+    result should containInAnyOrder(Seq(1, 2, 3))
+  }
+}
+
+"unionAll companion object" should "handle multiple collections" in {
+  runWithContext { sc =>
+    val sc1 = sc.parallelize(Seq(1, 2))
+    val sc2 = sc.parallelize(Seq(3, 4))
+    val sc3 = sc.parallelize(Seq(5, 6))
+    val result = SCollection.unionAll(Seq(sc1, sc2, sc3))
+    result should containInAnyOrder(Seq(1, 2, 3, 4, 5, 6))
+  }
+}
+"unionAll context method" should "handle empty collection safely" in {
+  runWithContext { sc =>
+    val empty = Seq.empty[SCollection[Int]]
+    val result = sc.unionAll(empty)
+    result should beEmpty
+  }
+}
+
+"unionAll context method" should "handle multiple collections" in {
+  runWithContext { sc =>
+    val sc1 = sc.parallelize(Seq(1, 2))
+    val sc2 = sc.parallelize(Seq(3, 4))
+    val result = sc.unionAll(Seq(sc1, sc2))
+    result should containInAnyOrder(Seq(1, 2, 3, 4))
+  }
+}

scio-core/src/main/scala/com/spotify/scio/values/ScioContextTest.scala

@@ -0,0 +1,16 @@
+"ScioContext.unionAll" should "handle empty collection safely" in {
+  runWithContext { sc =>
+    val empty = Seq.empty[SCollection[Int]]
+    val result = sc.unionAll(empty)
+    result should beEmpty
+  }
+}
+
+"ScioContext.unionAll" should "handle multiple collections" in {
+  runWithContext { sc =>
+    val sc1 = sc.parallelize(Seq(1, 2))
+    val sc2 = sc.parallelize(Seq(3, 4))
+    val result = sc.unionAll(Seq(sc1, sc2))
+    result should containInAnyOrder(Seq(1, 2, 3, 4))
+  }
+}