GH-8757: Add nullabilty to .support.converter

Fixes https://github.com/spring-projects/spring-integration/issues/8757

* Add ctor `ObjectStringMessageConverter(Charset)`
* create tests for `ObjectStringMessageConverter`
* fix codestyle issues
* revise JavaDoc
* `@NonNullApi` for `org.springframework.integration.support.converter`
* revise `SimpleMessageConverter`
* `inboundMessageMapper` and `outboundMessageMapper` are initialized to their default implementations
Bboth are set in non-default ctors with final setters
* remove `@Nullable` from `private BeanFactory beanFactory`
* Handle empty `String... headerNames`
* Some code clean up

spring-integration-core/src/test/java/org/springframework/integration/support/converter/ObjectStringMessageConverterTests.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed 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
+ *
+ *      https://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.springframework.integration.support.converter;
+
+
+import java.nio.charset.StandardCharsets;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.integration.support.MessageBuilder;
+import org.springframework.messaging.Message;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Falk Hanisch
+ *
+ * @since 6.2
+ */
+class ObjectStringMessageConverterTests {
+
+	private static final ArbitraryPayload PAYLOAD = new ArbitraryPayload(1234, "ÄÖÜ");
+
+	@Test
+	void testFromMessage() {
+		Message<ArbitraryPayload> message = MessageBuilder.withPayload(PAYLOAD).build();
+		ObjectStringMessageConverter converter = new ObjectStringMessageConverter();
+		Object result = converter.fromMessage(message, String.class);
+
+		assertThat(message.getPayload()).isEqualTo(PAYLOAD);
+		assertThat(result).isEqualTo(PAYLOAD.toString());
+
+		result = converter.fromMessage(message, Object.class);
+		assertThat(result).isNull();
+	}
+
+	@Test
+	void testToMessage() {
+		String text = PAYLOAD.text();
+		ObjectStringMessageConverter converter = new ObjectStringMessageConverter(StandardCharsets.ISO_8859_1);
+		Message<?> converted = converter.toMessage(text, null);
+
+		assertThat(converted).isNotNull();
+		assertThat(converted.getPayload()).isEqualTo(text.getBytes(StandardCharsets.ISO_8859_1));
+
+		converter.setSerializedPayloadClass(String.class);
+		converted = converter.toMessage(text, null);
+		assertThat(converted).isNotNull();
+		assertThat(converted.getPayload()).isEqualTo(text);
+
+		converted = converter.toMessage(PAYLOAD, null);
+		assertThat(converted).isNull();
+	}
+
+	private record ArbitraryPayload(Integer id, String text) {
+	}
+
+}