Add nullability annotations to module/spring-boot-reactor

See gh-46587

Author: mhalbritter

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

@@ -17,4 +17,7 @@
 /**
  * Auto-configuration for Reactor.
  */
+@NullMarked
 package org.springframework.boot.reactor.autoconfigure;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-reactor/src/main/java/org/springframework/boot/reactor/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Support classes for Reactor integration.
  */
+@NullMarked
 package org.springframework.boot.reactor;
+
+import org.jspecify.annotations.NullMarked;