fix 1652: rename NDR to empty senders in GUI

Bug 1652 indicates a slight misalignment of our use of NDR with the
expectations users might have:
We use it throughout our codebase meaning - mails with empty senders (NDRs
a.k.a bounces do most often have an empty sender (to prevent mailloops) but
quite a few messages being directly non-delivery-notifications also use an
empty sender (since noone would be reading the automated bounces for them) -
e.g. our quarantine report)

By renaming the GUI-display field it should be more clear to users of the
Tracking center.

The fix is left at the GUI-level, since the use of ndr for empty sender is
quite consistent throughout our complete stack (in that case from the GUI, via
pmg-api down to the pmg-log-tracker) and I don't see the direct benefit for
renaming it everywhere (and am lacking a better concise phrase for "empty
envelope sender").

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

js/MailTracker.js

@@ -123,7 +123,7 @@ Ext.define('PMG.MailTrackerFilter', {
 		    border: false,
 		    items: [
 			{
-			    boxLabel: gettext('Include NDRs'),
+			    boxLabel: gettext('Include Empty Senders'),
 			    xtype: 'proxmoxcheckbox',
 			    listeners: { change: 'onFilterChange' },
 			    reference: 'ndr',