feat: Null-mark the 'logmanager' package.

Change-Id: I42c3e1db77c6a30966c045faa492103c0946dedc
diff --git a/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/logmanager/package-info.java b/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/logmanager/package-info.java
index 3b6f38c..d36dfc1 100644
--- a/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/logmanager/package-info.java
+++ b/core/src/main/java/eu/mulk/quarkus/googlecloud/jsonlogging/logmanager/package-info.java
@@ -19,4 +19,7 @@
  * when used in conjunction with frameworks other than Quarkus (such as Spring Boot). See the class
  * documentation for details.
  */
+@NullMarked
 package eu.mulk.quarkus.googlecloud.jsonlogging.logmanager;
+
+import org.jspecify.annotations.NullMarked;