[SPARK-25371][SQL] struct() should allow being called with 0 args

## What changes were proposed in this pull request?

SPARK-21281 introduced a check for the inputs of `CreateStructLike` to be non-empty. This means that `struct()`, which was previously considered valid, now throws an Exception.  This behavior change was introduced in 2.3.0. The change may break users' application on upgrade and it causes `VectorAssembler` to fail when an empty `inputCols` is defined.

The PR removes the added check making `struct()` valid again.

## How was this patch tested?

added UT

Closes #22373 from mgaido91/SPARK-25371.

Authored-by: Marco Gaido <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: mgaido91

mllib/src/test/scala/org/apache/spark/ml/feature/VectorAssemblerSuite.scala

@@ -256,4 +256,9 @@ class VectorAssemblerSuite
     assert(runWithMetadata("keep", additional_filter = "id1 > 2").count() == 4)
   }
 
+  test("SPARK-25371: VectorAssembler with empty inputCols") {
+    val vectorAssembler = new VectorAssembler().setInputCols(Array()).setOutputCol("a")
+    val output = vectorAssembler.transform(dfWithNullsAndNaNs)
+    assert(output.select("a").limit(1).collect().head == Row(Vectors.sparse(0, Seq.empty)))
+  }
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypeCreator.scala

@@ -379,10 +379,7 @@ trait CreateNamedStructLike extends Expression {
   }
 
   override def checkInputDataTypes(): TypeCheckResult = {
-    if (children.length < 1) {
-      TypeCheckResult.TypeCheckFailure(
-        s"input to function $prettyName requires at least one argument")
-    } else if (children.size % 2 != 0) {
+    if (children.size % 2 != 0) {
       TypeCheckResult.TypeCheckFailure(s"$prettyName expects an even number of arguments.")
     } else {
       val invalidNames = nameExprs.filterNot(e => e.foldable && e.dataType == StringType)

sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala

@@ -2677,8 +2677,6 @@ class DataFrameFunctionsSuite extends QueryTest with SharedSQLContext {
     val funcsMustHaveAtLeastOneArg =
       ("coalesce", (df: DataFrame) => df.select(coalesce())) ::
       ("coalesce", (df: DataFrame) => df.selectExpr("coalesce()")) ::
-      ("named_struct", (df: DataFrame) => df.select(struct())) ::
-      ("named_struct", (df: DataFrame) => df.selectExpr("named_struct()")) ::
       ("hash", (df: DataFrame) => df.select(hash())) ::
       ("hash", (df: DataFrame) => df.selectExpr("hash()")) :: Nil
     funcsMustHaveAtLeastOneArg.foreach { case (name, func) =>