proper comparison of Option values, following hint by IntelliJ;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Wed Jan 15 16:18:24 2020 +0100
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Wed Jan 15 19:45:06 2020 +0100
@@ -116,7 +116,7 @@
 
     def known_versions(rev: String, afp_rev: Option[String]): Boolean =
       known && rev != "" && isabelle_version == rev &&
-      (afp_rev.isEmpty || afp_rev.get != "" && afp_version == afp_rev.get)
+      (afp_rev.isEmpty || afp_rev.get != "" && afp_version == afp_rev)
   }
 
   def recent_items(db: SQL.Database,