Color randomize doesn't touch opacity if opacity is unset

Continuation of #234 (closed)

Steps to reproduce:

• open Inkscape
• create a rectangle
• select it
• go to XML Editor and make sure fill is set, but opacity/fill-opacity/stroke-opacity are not. if using default preferences, you can skip this step
• go to extension > color > randomize
• set all sliders to 0
• change opacity to 100

What happened?

• opacity only changes if opacity is already set.
• note that previously only opacity was changed (change is not necessarily bad though)

Version info

• 1e560830 in Inkscape 1.1-dev (d1157cc2fb, 2020-05-29, custom) Linux Mint 19.1
• also experienced in https://chat.inkscape.org/channel/team_testing?msg=nunFSGBftep4tmiew