Fix UpgradeTool execution log (#3614)

It seems that in some cases GSON is not able to correctly serialize/deserialize ZonedDateTime. Since we do not depend on the value in any case (it's just informational), we can store a String instead.

A missing metadata database also failed the check when upgrading items. This has been fixed, too.

Signed-off-by: Jan N. Klug <github@klug.nrw>

tools/upgradetool/src/main/java/org/openhab/core/tools/internal/Upgrader.java

@@ -78,7 +78,8 @@ public void itemCopyUnitToMetadata() {
             logger.error("Cannot access item database '{}', check path and access rights.", itemJsonDatabasePath);
             return;
         }
-        if (!Files.isWritable(metadataJsonDatabasePath)) {
+        // missing metadata database is also fine, we create one in that case
+        if (!Files.isWritable(metadataJsonDatabasePath) && Files.exists(metadataJsonDatabasePath)) {
             logger.error("Cannot access metadata database '{}', check path and access rights.",
                     metadataJsonDatabasePath);
             return;
@@ -164,10 +165,10 @@ public void linkUpgradeJsProfile() {
     }
 
     private static class UpgradeRecord {
-        public final ZonedDateTime executionDate;
+        public final String executionDate;
 
         public UpgradeRecord(ZonedDateTime executionDate) {
-            this.executionDate = executionDate;
+            this.executionDate = executionDate.toString();
         }
     }
 }