Spark: Add regression test for MERGE struct nullification

spark/v4.1/spark-extensions/src/test/java/org/apache/iceberg/spark/extensions/TestMerge.java

@@ -3001,6 +3001,29 @@ public void testMergeToWapBranchWithTableBranchIdentifier() {
         });
   }
 
+  @TestTemplate
+  public void testMergePreservesNullableStructWithRequiredChildren() {
+    // Regression test for https://github.com/apache/iceberg/issues/16246
+    // A nullable struct with non-nullable children should not have its children
+    // silently nullified when a MERGE updates a different column.
+    createAndInitTable(
+        "id INT, status STRING, info STRUCT<type:STRING NOT NULL, attr:STRING NOT NULL>",
+        "{ \"id\": 1, \"status\": \"active\", \"info\": { \"type\": \"A\", \"attr\": \"val1\" } }");
+    createOrReplaceView("source", "{ \"id\": 1, \"status\": \"inactive\" }");
+
+    sql(
+        "MERGE INTO %s t USING source s "
+            + "ON t.id == s.id "
+            + "WHEN MATCHED THEN "
+            + "  UPDATE SET t.status = s.status",
+        commitTarget());
+
+    assertEquals(
+        "Struct children should be preserved after MERGE updates a different column",
+        ImmutableList.of(row(1, "inactive", row("A", "val1"))),
+        sql("SELECT * FROM %s", selectTarget()));
+  }
+
   private void checkJoinAndFilterConditions(String query, String join, String icebergFilters) {
     // disable runtime filtering for easier validation
     withSQLConf(