Background color changes do not take effect immediately when color-style property is not yet written

Issue Description

When there are no values in xfce4-desktop/backdrop/, the Desktop settings dialog defaults to "Solid color" in the Color dropdown. If you disable wallpapers and then configure a color, the color is not reflected on the desktop until you change the selection in the Color dropdown.

This is confusing when configuring Xfce for the first time after a new installation.

Steps to Reproduce Issue

1. Clear out xfce4-desktop/backdrop/ settings for the current screen
2. Open Desktop settings
   • Notice that "Solid color" is selected in the Color dropdown
3. Select "None" in the Style dropdown
4. Set the first color (rgba1) to red
   • Notice that the desktop background is still black
   • The xfce4-desktop/backdrop/screen0/monitorVirtual-1/workspace0/color-style property is not yet present
5. Select "Vertical gradient" in the Color dropdown
   • The xfce4-desktop/backdrop/screen0/monitorVirtual-1/workspace0/color-style property is now present
6. Reselect "Solid color" in the Color dropdown
   • The desktop background is now red

Expected Behavior

I would expect a color change to be reflected immediately if "Solid color" is selected in the dialog

Additional Information

Screenshot after step 4:

Environment & Versions

• OS distribution and version: Arch Linux
• Overall Xfce version: 4.20
• xfdesktop version (git hash if not a release): d58b66ff
• Windowing system (X11/Wayland): X11
• Compositor and version (if Wayland): N/A
• GTK Theme: Adwaita
• Icon Theme: elementary
• UI Scale Factor: 1x