ui: tape/BackupOverview: do not iterate over empty tapes

tapes not belonging to a pool cannot contain data, so skip them

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

www/tape/BackupOverview.js

@@ -46,6 +46,9 @@ Ext.define('PBS.TapeManagement.BackupOverview', {
 
 	    for (const entry of content_response.result.data) {
 		let pool = entry.pool;
+		if (pool === undefined) {
+		    continue; // pools not belonging to a pool cannot contain data
+		}
 		let media_set = entry['media-set-name'];
 		if (data[pool] === undefined) {
 		    data[pool] = {};