Check for Spring Data before configuring Couchbase

Add guard to `CouchbaseAutoConfiguration` so ensure that Spring Data
Couchbase is on the classpath.

Fixes gh-7453

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/couchbase/CouchbaseAutoConfiguration.java

@@ -44,7 +44,7 @@ import org.springframework.data.couchbase.config.CouchbaseConfigurer;
  * @since 1.4.0
  */
 @Configuration
-@ConditionalOnClass({ CouchbaseBucket.class, Cluster.class })
+@ConditionalOnClass({ CouchbaseBucket.class, Cluster.class, CouchbaseConfigurer.class })
 @Conditional(CouchbaseAutoConfiguration.CouchbaseCondition.class)
 @EnableConfigurationProperties(CouchbaseProperties.class)
 public class CouchbaseAutoConfiguration {