[iceberg] Make Iceberg partition a required field, as specified in Iceberg spec (#5386)

Author: siadat

paimon-core/src/main/java/org/apache/paimon/iceberg/manifest/IcebergDataFileMeta.java

@@ -218,7 +218,7 @@ public static RowType schema(RowType partitionType) {
         fields.add(new DataField(134, "content", DataTypes.INT().notNull()));
         fields.add(new DataField(100, "file_path", DataTypes.STRING().notNull()));
         fields.add(new DataField(101, "file_format", DataTypes.STRING().notNull()));
-        fields.add(new DataField(102, "partition", partitionType));
+        fields.add(new DataField(102, "partition", partitionType.notNull()));
         fields.add(new DataField(103, "record_count", DataTypes.BIGINT().notNull()));
         fields.add(new DataField(104, "file_size_in_bytes", DataTypes.BIGINT().notNull()));
         fields.add(

paimon-core/src/test/java/org/apache/paimon/iceberg/manifest/IcebergDataFileMetaTest.java

@@ -0,0 +1,48 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.paimon.iceberg.manifest;
+
+import org.apache.paimon.TestKeyValueGenerator;
+import org.apache.paimon.types.DataField;
+import org.apache.paimon.types.RowType;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class IcebergDataFileMetaTest {
+
+    @Test
+    @DisplayName("Test partition is a required field")
+    void testPartitionIsNotNull() {
+        RowType partitionType = TestKeyValueGenerator.SINGLE_PARTITIONED_PART_TYPE;
+        RowType schema = IcebergDataFileMeta.schema(partitionType);
+        List<DataField> fields = schema.getFields();
+
+        Optional<DataField> partitionField = fields.stream().filter(f -> f.id() == 102).findFirst();
+
+        assertThat(partitionField).isPresent();
+        assertThat(partitionField.get().name()).isEqualTo("partition");
+        assertThat(partitionField.get().type()).isEqualTo(partitionType.notNull());
+    }
+}