Add nullability annotations to module/spring-boot-zipkin

See gh-46587

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

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

module/spring-boot-zipkin/src/main/java/org/springframework/boot/zipkin/docker/compose/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Support for Docker Compose Zipkin service connections.
  */
+@NullMarked
 package org.springframework.boot.zipkin.docker.compose;
+
+import org.jspecify.annotations.NullMarked;

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

@@ -17,4 +17,7 @@
 /**
  * Support for testcontainers Zipkin service connections.
  */
+@NullMarked
 package org.springframework.boot.zipkin.testcontainers;
+
+import org.jspecify.annotations.NullMarked;