Add nullability annotations to module/spring-boot-session-data-mongodb

See gh-46587

Author: mhalbritter

module/spring-boot-session-data-mongodb/src/main/java/org/springframework/boot/session/data/mongodb/autoconfigure/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Auto-configuration for Spring Session Data MongoDB.
  */
+@NullMarked
 package org.springframework.boot.session.data.mongodb.autoconfigure;
+
+import org.jspecify.annotations.NullMarked;