Fix Nullability Error

src/main/java/org/mybatis/dynamic/sql/util/ValueWhenPresentMapping.java

@@ -35,11 +35,11 @@ private ValueWhenPresentMapping(SqlColumn<T> column, Supplier<@Nullable T> value
     }
 
     public Optional<Object> value() {
-        return Optional.ofNullable(valueSupplier.get()).map(this::convert);
+        return Optional.ofNullable(valueSupplier.get()).flatMap(this::convert);
     }
 
-    private @Nullable Object convert(@Nullable T value) {
-        return localColumn.convertParameterType(value);
+    private Optional<Object> convert(T value) {
+        return Optional.ofNullable(localColumn.convertParameterType(value));
     }
 
     @Override

src/test/java/examples/spring/LastNameParameterConverter.java

@@ -16,14 +16,19 @@
 package examples.spring;
 
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 import org.mybatis.dynamic.sql.ParameterTypeConverter;
 import org.springframework.core.convert.converter.Converter;
 
 @NullMarked
 public class LastNameParameterConverter implements ParameterTypeConverter<LastName, String>,
         Converter<LastName, String> {
     @Override
-    public String convert(LastName source) {
-        return source.getName();
+    public @Nullable String convert(LastName source) {
+        if ("Slate".equals(source.getName())) {
+            return null;
+        } else {
+            return source.getName();
+        }
     }
 }

src/test/java/examples/spring/PersonTemplateTest.java

@@ -31,6 +31,8 @@
 import org.mybatis.dynamic.sql.insert.GeneralInsertModel;
 import org.mybatis.dynamic.sql.insert.InsertModel;
 import org.mybatis.dynamic.sql.insert.MultiRowInsertModel;
+import org.mybatis.dynamic.sql.insert.render.GeneralInsertStatementProvider;
+import org.mybatis.dynamic.sql.render.RenderingStrategies;
 import org.mybatis.dynamic.sql.select.SelectModel;
 import org.mybatis.dynamic.sql.update.UpdateModel;
 import org.mybatis.dynamic.sql.util.Buildable;
@@ -379,6 +381,32 @@ void testInsertSelective() {
         assertThat(rows).isEqualTo(1);
     }
 
+    @Test
+    void testGeneralInsertWhenTypeConverterReturnsNull() {
+
+        GeneralInsertStatementProvider insertStatement = insertInto(person)
+                .set(id).toValue(100)
+                .set(firstName).toValue("Joe")
+                .set(lastName).toValueWhenPresent(LastName.of("Slate"))
+                .set(birthDate).toValue(new Date())
+                .set(employed).toValue(true)
+                .set(occupation).toValue("Quarry Owner")
+                .set(addressId).toValue(1)
+                .build()
+                .render(RenderingStrategies.SPRING_NAMED_PARAMETER);
+
+        assertThat(insertStatement.getInsertStatement())
+                .isEqualTo("insert into Person (id, first_name, birth_date, employed, occupation, address_id) values (:p1, :p2, :p3, :p4, :p5, :p6)");
+        int rows = template.generalInsert(insertStatement);
+        assertThat(rows).isEqualTo(1);
+
+        Buildable<SelectModel> selectStatement = select(id, firstName, lastName, birthDate, employed, occupation, addressId)
+                .from(person)
+                .where(id, isEqualTo(100));
+        Optional<PersonRecord> newRecord = template.selectOne(selectStatement, personRowMapper);
+        assertThat(newRecord).hasValueSatisfying(r -> assertThat(r.getLastName().getName()).isNull());
+    }
+
     @Test
     void testUpdateByPrimaryKey() {
 

src/test/resources/examples/simple/CreateSimpleDB.sql

@@ -30,7 +30,7 @@ create table Address (
 create table Person (
    id int not null,
    first_name varchar(30) not null,
-   last_name varchar(30) not null,
+   last_name varchar(30) null,
    birth_date date not null,
    employed varchar(3) not null,
    occupation varchar(30) null,