We translate all pure gray colors into cmyk having c,m,y=0 and only
the k value expressing the darkness. But a fix introduced to avoid
division by 0 caused rgb(0, 0, 0) to be an exception to this; it ended
up being translated as c,m,y,k=1 instead.
Fix by catching the potential div-by-0 situation earlier and directly
set the orthodox cmyk translation: c,m,y=0,k=1.
Fixes: QTBUG-73171
Change-Id: I3774eaf9d96e096ac5c47c55d28881bea2bd1309
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
01f090e9e4