[SPARK-32817][SQL] DPP throws error when broadcast side is empty

### What changes were proposed in this pull request?

In `SubqueryBroadcastExec.relationFuture`, if the `broadcastRelation` is an `EmptyHashedRelation`, then `broadcastRelation.keys()` will throw `UnsupportedOperationException`.

### Why are the changes needed?

To fix a bug.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Added a new test.

Closes #29671 from wzhfy/dpp_empty_broadcast.

Authored-by: Zhenhua Wang <wzh_zju@163.com>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>

Author: wzhfy

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashedRelation.scala

@@ -1091,7 +1091,7 @@ case object EmptyHashedRelation extends HashedRelation {
   override def keyIsUnique: Boolean = true
 
   override def keys(): Iterator[InternalRow] = {
-    throw new UnsupportedOperationException
+    Iterator.empty
   }
 
   override def close(): Unit = {}

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

@@ -1344,6 +1344,23 @@ abstract class DynamicPartitionPruningSuiteBase
       }
     }
   }
+
+  test("SPARK-32817: DPP throws error when the broadcast side is empty") {
+    withSQLConf(
+      SQLConf.DYNAMIC_PARTITION_PRUNING_ENABLED.key -> "true",
+      SQLConf.DYNAMIC_PARTITION_PRUNING_REUSE_BROADCAST_ONLY.key -> "true") {
+      val df = sql(
+        """
+          |SELECT * FROM fact_sk f
+          |JOIN dim_store s
+          |ON f.store_id = s.store_id WHERE s.country = 'XYZ'
+        """.stripMargin)
+
+      checkPartitionPruningPredicate(df, false, true)
+
+      checkAnswer(df, Nil)
+    }
+  }
 }
 
 class DynamicPartitionPruningSuiteAEOff extends DynamicPartitionPruningSuiteBase {

sql/core/src/test/scala/org/apache/spark/sql/execution/joins/HashedRelationSuite.scala

@@ -621,7 +621,7 @@ class HashedRelationSuite extends SharedSparkSession {
     }
   }
 
-  test("EmptyHashedRelation return null in get / getValue") {
+  test("EmptyHashedRelation override methods behavior test") {
     val buildKey = Seq(BoundReference(0, LongType, false))
     val hashed = HashedRelation(Seq.empty[InternalRow].toIterator, buildKey, 1, mm)
     assert(hashed == EmptyHashedRelation)
@@ -631,6 +631,10 @@ class HashedRelationSuite extends SharedSparkSession {
     assert(hashed.get(key) == null)
     assert(hashed.getValue(0L) == null)
     assert(hashed.getValue(key) == null)
+
+    assert(hashed.keys().isEmpty)
+    assert(hashed.keyIsUnique)
+    assert(hashed.estimatedSize == 0)
   }
 
   test("SPARK-32399: test methods related to key index") {