HHH-10360 : Natural ID columns are nullable by default

(cherry picked from commit 10a035481d)

hibernate-core/src/main/java/org/hibernate/boot/model/source/internal/hbm/ModelBinder.java

@@ -1889,7 +1889,7 @@ public class ModelBinder {
 				sourceDocument,
 				attributeSource.getRelationalValueSources(),
 				value,
-				true,
+				attributeSource.areValuesNullableByDefault(),
 				new RelationalObjectBinder.ColumnNamingDelegate() {
 					@Override
 					public Identifier determineImplicitName(LocalMetadataBuildingContext context) {

hibernate-core/src/main/java/org/hibernate/boot/model/source/internal/hbm/SingularAttributeSourceBasicImpl.java

@@ -171,7 +171,10 @@ class SingularAttributeSourceBasicImpl
 
 	@Override
 	public boolean areValuesNullableByDefault() {
-		return ! Helper.getValue( propertyElement.isNotNull(), false );
+		return ! Helper.getValue(
+				propertyElement.isNotNull(),
+				naturalIdMutability != NaturalIdMutability.NOT_NATURAL_ID
+		);
 	}
 
 	@Override

hibernate-core/src/main/java/org/hibernate/boot/model/source/internal/hbm/SingularAttributeSourceManyToOneImpl.java

@@ -216,7 +216,7 @@ class SingularAttributeSourceManyToOneImpl
 
 	@Override
 	public boolean areValuesNullableByDefault() {
-		return true;
+		return getNaturalIdMutability() == NaturalIdMutability.NOT_NATURAL_ID;
 	}
 
 	@Override