Fix combobox initial sizing

Summary:
We have a listview in a popup. The popup is resized to the listview
content height.

Because the popup starts out at size 0 the listview thinks it's too
small to show anything.

So it doesn't load any delegates.

As it has no delegates it can't guess the contentHeight.
contentHeight is a guess based on count * (currentHeight
/ currentlyLoadedDelegates)

That means the popup doesn't resize properly.

Ultimately it does end up showing a two entries, but it doesn't
re-evaluate the contentHeight properly to resize the popup correctly.

Forcing it to load one delegate is enough to break that cycle.

BUG: 403736

Test Plan:
Opened popup in cursortheme and fonts
Worked correctly

Reviewers: #plasma, mart, ngraham

Reviewed By: #plasma, mart, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18597