Add nullability annotations to module/spring-boot-data-cassandra

See gh-46587

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

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