Allow use of null in REST Client request body

extensions/resteasy-reactive/rest-client-reactive/deployment/src/test/java/io/quarkus/rest/client/reactive/NullBodyTest.java

@@ -0,0 +1,45 @@
+package io.quarkus.rest.client.reactive;
+
+import java.net.URI;
+
+import jakarta.ws.rs.POST;
+import jakarta.ws.rs.Path;
+
+import org.assertj.core.api.Assertions;
+import org.eclipse.microprofile.rest.client.RestClientBuilder;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import io.quarkus.test.QuarkusUnitTest;
+import io.quarkus.test.common.http.TestHTTPResource;
+
+public class NullBodyTest {
+
+    @RegisterExtension
+    static final QuarkusUnitTest config = new QuarkusUnitTest()
+            .withApplicationRoot((jar) -> jar.addClasses(Client.class));
+
+    @TestHTTPResource
+    URI baseUri;
+
+    @Test
+    void withBody() {
+        Client client = RestClientBuilder.newBuilder().baseUri(baseUri).build(Client.class);
+
+        Assertions.assertThatThrownBy(() -> client.call("test")).hasMessageContaining("404");
+    }
+
+    @Test
+    void withoutBody() {
+        Client client = RestClientBuilder.newBuilder().baseUri(baseUri).build(Client.class);
+
+        Assertions.assertThatThrownBy(() -> client.call(null)).hasMessageContaining("404");
+    }
+
+    public interface Client {
+
+        @Path("/")
+        @POST
+        void call(String body);
+    }
+}

independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/RestClientRequestContext.java

@@ -241,6 +241,10 @@ public ClientRequestContextImpl getOrCreateClientRequestContext() {
     public Buffer writeEntity(Entity<?> entity, MultivaluedMap<String, String> headerMap, WriterInterceptor[] interceptors)
             throws IOException {
         Object entityObject = entity.getEntity();
+        if (entityObject == null) {
+            return AsyncInvokerImpl.EMPTY_BUFFER;
+        }
+
         Class<?> entityClass;
         Type entityType;
         if (entityObject instanceof GenericEntity) {