Don't show splash screen in shell mode

Summary:

When using Inkscape's (excellent) --shell mode for headless tasks, the splash screen is shown and remains visible until the process exits. I'd expect this not to be the case when using it from automation.

Steps to reproduce:

Run inkscape --shell

What happened?

The splash screen opened and remained open until I exited the shell.

What should have happened?

The splash screen should not have been displayed at any point.

Version info

Inkscape 1.4.1 (93de688d07, 2025-03-30)

                      Compile  (Run)
    GLib version:     2.84.1
    GTK version:      3.24.49 (3.24.49)
    glibmm version:   2.66.8
    gtkmm version:    3.24.10
    libxml2 version:  2.13.7
    libxslt version:  1.1.43
    Cairo version:    1.18.4 (1.18.4)
    Pango version:    1.56.3 (1.56.3)
    HarfBuzz version: 11.0.1 (11.2.0)

    OS version:       Arch Linux