consider preferredSize(s) only for >0 values

--do not consider preferredWidth or preferredHeight
for applets when that value is <=0 otherwise that
applet it may become invisible. Such case is the
plasma analog clock when it is locked at a vertical
dock

BUG: 398098
FIXED-IN: 0.8.1
(cherry picked from commit ec0e16ca5a16809e1ce947a80dc6f7e71a093338)