QuickDesigner: Updating the preview for setting colors to transparent fails for implicit color

Steps to reproduce:

• create/open Qt Quick UI / Application, switch to Design mode
• drag a Rectangle onto the preview
• increment the border width of the Rectangle (border has a default color of black - not implicitly set)
• switch border color to transparent
  • preview is not updated
• switch back to solid color
  • change/explicitly set a color
• switch to transparent mode and back to solid
  • preview is updated

Expected result:

• even for colors that aren't set explicitly transparency is updated inside the preview