dashboard: fix datastore full estimate for not changing usage

If the usage of a datastore did not change, we did not
return an estimate. The ui interpreted this as 'not enough data', but
it should actually be 'never'.

Fixing this by always setting the estimate first to 0 and overwriting
if we successfully calculated one, and checking for 'undefined' in the ui.

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

www/Utils.js

@@ -277,7 +277,7 @@ Ext.define('PBS.Utils', {
     },
 
     render_estimate: function(value) {
-	if (!value) {
+	if (value === undefined) {
 	    return gettext('Not enough data');
 	}