Replaced hard-coded value for user constructor

with email property from gitlab4j.api.models.User

hard coded values might not be appropriate for all cases

may cause issues in the application

src/main/java/org/jenkinsci/plugins/GitLabOAuthUserDetails.java

@@ -12,7 +12,7 @@ public class GitLabOAuthUserDetails extends User {
     private static final long serialVersionUID = 1709511212188366292L;
 
     public GitLabOAuthUserDetails(org.gitlab4j.api.models.User user, GrantedAuthority[] authorities) {
-        super(user.getUsername(), "", true, true, true, true, authorities);
+        super(user.getUsername(), user.getEmail(), true, true, true, true, authorities);
     }
 
 }