Fix applet centering by spacers

Prior to this fix, when using spacers to center an applet in a panel,
the applet would be centered in the available space. However, the
desired behavior is for the applet to be centered in the middle of the
panel, just as it was in Plasma 5.27.

The reason of this bug are the API changes in KF6. Prior to this fix,
the code would try to call nonexistent properties, resulting in the
optimalSize property being NaN.

BUG: 476383