--use-dithering option causes empty PNG files when Inkscape build does not support dithering

Summary:

Using the ppa Inkscape version which is built without dithering support, I get invalid results when using the option.

Steps to reproduce:

• on the command line, use inkscape --export-type=png --export-filename=/tmp/dithered.png --export-png-use-dithering /tmp/test.svg

What happened?

• an all transparent png image is exported:

What should have happened?

Inkscape should warn that the build does not support dithering and quit.

Version info

Inkscape 1.2 (1:1.2+202206011327+fc4e4096c5)

    GLib version:     2.64.6
    GTK version:      3.24.20
    glibmm version:   2.64.2
    gtkmm version:    3.24.2
    libxml2 version:  2.9.10
    libxslt version:  1.1.34
    Cairo version:    1.16.0
    Pango version:    1.44.7
    HarfBuzz version: 2.6.4

    OS version:       Linux Mint 20.3