Merge pull request #21769 from tszmytka

* pr/21769:
  Back-off if a custom ReactiveSessionFactory is defined

Closes gh-21769

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/data/cassandra/CassandraReactiveDataAutoConfiguration.java

@@ -55,6 +55,7 @@ public class CassandraReactiveDataAutoConfiguration {
 	}
 
 	@Bean
+	@ConditionalOnMissingBean
 	public ReactiveSessionFactory reactiveCassandraSessionFactory(ReactiveSession reactiveCassandraSession) {
 		return new DefaultReactiveSessionFactory(reactiveCassandraSession);
 	}