Avoid codec auto-configuration when WebFlux is not present

This commit prevents the `CodecsAutoConfiguration` from being processed
if spring-webflux is not on classpath, since it is only useful for the
WebFlux server infrastructure or `WebClient`.

Closes gh-15690

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/http/codec/CodecsAutoConfiguration.java

@@ -33,6 +33,7 @@ import org.springframework.http.codec.CodecConfigurer;
 import org.springframework.http.codec.json.Jackson2JsonDecoder;
 import org.springframework.http.codec.json.Jackson2JsonEncoder;
 import org.springframework.util.MimeType;
+import org.springframework.web.reactive.function.client.WebClient;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for
@@ -43,7 +44,7 @@ import org.springframework.util.MimeType;
  * @since 2.0.0
  */
 @Configuration(proxyBeanMethods = false)
-@ConditionalOnClass(CodecConfigurer.class)
+@ConditionalOnClass({ CodecConfigurer.class, WebClient.class })
 @AutoConfigureAfter(JacksonAutoConfiguration.class)
 public class CodecsAutoConfiguration {