use jspecify @nullable in AbstractMetadataRepository

this is the only use of @nullable in the fdp

Author: dennisvang

pom.xml

@@ -189,7 +189,6 @@
             <artifactId>spring-boot-starter-validation</artifactId>
         </dependency>
         <dependency>
-            <!-- with transitive dependency on hibernate-validator -->
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-validation-test</artifactId>
             <scope>test</scope>

src/main/java/org/fairdatateam/fairdatapoint/database/rdf/repository/AbstractMetadataRepository.java

@@ -22,7 +22,6 @@
  */
 package org.fairdatateam.fairdatapoint.database.rdf.repository;
 
-import jakarta.annotation.Nullable;
 import lombok.extern.slf4j.Slf4j;
 import org.eclipse.rdf4j.common.iteration.Iterations;
 import org.eclipse.rdf4j.model.*;
@@ -32,6 +31,7 @@
 import org.eclipse.rdf4j.repository.Repository;
 import org.eclipse.rdf4j.repository.RepositoryConnection;
 import org.eclipse.rdf4j.repository.RepositoryException;
+import org.jspecify.annotations.Nullable;
 
 import java.util.*;