plasmaquick/appletpopup: don't resize in response to min/max size with invalid sizes
There's two cases where this can go wrong:
- the current size is empty, because for example the full representation of an applet was never opened. This made AppletPopup::updateMinSize resize the window to the minimum size
- the maximum size is empty. This made AppletPopup::updateMaxSize resize the window to be an empty size
In both cases, the applet popup would wrongly resize away from what the user configured