Fix two skipped sentinel errors for builds against musl libc

My previous MR !3377 (merged) was not complet, I missed 2 sentinels. Sorry for that.

mentioned in merge request !3378 (merged)

merged

Hmmm, this fixed remaining linux sentinels but I just notticed windows pipelines hitting simmilar issues because they use GCC and -Wstrict-null-sentinel is applied as well. I can prepare MR with a fix for those as well, but I don't have any windows machine around me to test it. Unfortunately, CI does not run in my fork because of latest gitlab policy, but maybe you can somehow run it from MR before merge to fix it completely in, hopefully, last MR related to this :)

Looking at https://gitlab.com/inkscape/inkscape/-/pipelines/331473034 I think we're good or did you spot something I missed?

Ah, never mind, I missed a third MR that landed in the meantime: !3380 (merged)

Yes, everything seems to be fixed in master now. I'll fix the backport MR to 1.1.x later.