klipper: fix copying for the first time not copying anything with MaxClipItems=1
Reason for the change
When inserting a new clip with MaxClipItems=1, in the slot for HistoryModel::changed, if the only existing clip item is removed, the clipboard will be reset, which is unnecessary and will break the clipboard.
Test plan
Screenshots or screen recordings
Bugs fixed
BUG: 417590 FIXED-IN: 6.3