Do not assert if we ask dimensions of a file with no ink box

Avoid to assert on files that do not have ink box at all.
Also add a testcase and improve the test.
Hopefully fixes GNOME/librsvg#324:
at least it fixes the mininal test case, but the crash of vmware
viewer needs to be tested.

Fixes GNOME/librsvg#324
3 jobs for issue324 in 7 minutes and 57 seconds (queued for 4 minutes and 54 seconds)
Status Job ID Name Coverage
  Test
passed #91770
fedora test

00:07:36

 
  Lint
manual #91772
allowed to fail manual
clippy
passed #91771
rustfmt

00:00:20