Fix sentinel error in dbus extension

Petr Vaněkrequested to merge
arkamar/inkscape:sentinel-fix into master

Another sentinel was missed in dbus extension, because it is not covered in CI and it was not covered in my tests either.

Fixes: 2170e8a3 ("Enable -Wstrict-null-sentinel by default for GCC") See-also: !3377 (merged)

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