AbstractMessageConverter delegates null headers

See gh-29768

spring-messaging/src/main/java/org/springframework/messaging/converter/AbstractMessageConverter.java

@@ -251,7 +251,7 @@ public abstract class AbstractMessageConverter implements SmartMessageConverter
 
 	@Nullable
 	protected MimeType getMimeType(@Nullable MessageHeaders headers) {
-		return (headers != null && this.contentTypeResolver != null ? this.contentTypeResolver.resolve(headers) : null);
+		return (this.contentTypeResolver != null ? this.contentTypeResolver.resolve(headers) : null);
 	}
 
 	/**

spring-messaging/src/test/java/org/springframework/messaging/converter/DefaultContentTypeResolverTests.java

@@ -97,4 +97,12 @@ public class DefaultContentTypeResolverTests {
 		assertThat(this.resolver.resolve(headers)).isEqualTo(MimeTypeUtils.APPLICATION_JSON);
 	}
 
+	@Test
+	public void resolveDefaultMimeTypeWithNoHeader() {
+		this.resolver.setDefaultMimeType(MimeTypeUtils.APPLICATION_JSON);
+		MessageHeaders headers = null;
+
+		assertThat(this.resolver.resolve(headers)).isEqualTo(MimeTypeUtils.APPLICATION_JSON);
+	}
+
 }

spring-messaging/src/test/java/org/springframework/messaging/converter/MessageConverterTests.java

@@ -138,6 +138,18 @@ public class MessageConverterTests {
 		assertThat(message.getHeaders().get(MessageHeaders.CONTENT_TYPE)).isEqualTo(MimeTypeUtils.TEXT_PLAIN);
 	}
 
+	@Test
+	public void toMessageDefaultContentType() {
+		DefaultContentTypeResolver resolver = new DefaultContentTypeResolver();
+		resolver.setDefaultMimeType(MimeTypeUtils.TEXT_PLAIN);
+		TestMessageConverter converter = new TestMessageConverter();
+		converter.setContentTypeResolver(resolver);
+		converter.setStrictContentTypeMatch(true);
+
+		Message<?> message = converter.toMessage("ABC", null);
+		assertThat(message.getHeaders().get(MessageHeaders.CONTENT_TYPE)).isEqualTo(MimeTypeUtils.TEXT_PLAIN);
+	}
+
 
 	private static class TestMessageConverter extends AbstractMessageConverter {