Add missing @NullMarked

Closes gh-18514

web/src/main/java/org/springframework/security/web/jackson/DefaultSavedRequestMixin.java

@@ -19,6 +19,7 @@ package org.springframework.security.web.jackson;
 import com.fasterxml.jackson.annotation.JsonAutoDetect;
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import org.jspecify.annotations.Nullable;
 import tools.jackson.databind.annotation.JsonDeserialize;
 
 import org.springframework.security.web.savedrequest.DefaultSavedRequest;
@@ -40,6 +41,6 @@ import org.springframework.security.web.savedrequest.DefaultSavedRequest;
 abstract class DefaultSavedRequestMixin {
 
 	@JsonInclude(JsonInclude.Include.NON_NULL)
-	String matchingRequestParameterName;
+	@Nullable String matchingRequestParameterName;
 
 }

web/src/main/java/org/springframework/security/web/jackson/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Jackson 3+ serialization support for web.
  */
+@NullMarked
 package org.springframework.security.web.jackson;
+
+import org.jspecify.annotations.NullMarked;