Ensure that MongoSessionConfiguration backs off when classes are missing

Closes gh-9552

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/MongoSessionConfiguration.java

@@ -18,6 +18,7 @@ package org.springframework.boot.autoconfigure.session;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Conditional;
@@ -33,6 +34,7 @@ import org.springframework.session.data.mongo.config.annotation.web.http.MongoHt
  * @author Stephane Nicoll
  */
 @Configuration
+@ConditionalOnClass(MongoHttpSessionConfiguration.class)
 @ConditionalOnMissingBean(SessionRepository.class)
 @ConditionalOnBean(MongoOperations.class)
 @Conditional(SessionCondition.class)