Fix assumption that all layer artists have a zorder

[astrofrog]

I think it is sufficient to ignore the callback on state.zorder which should exist.

[codecov]

Codecov Report

Merging #2226 (8e128fa) into master (5978a92) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2226   +/-   ##
=======================================
  Coverage   88.03%   88.04%           
=======================================
  Files         247      247           
  Lines       23168    23164    -4     
=======================================
- Hits        20396    20394    -2     
+ Misses       2772     2770    -2     

Impacted Files	Coverage Δ
glue/app/qt/layer_tree_widget.py	80.59% <ø> (+0.36%)	⬆️
glue/viewers/common/viewer.py	90.76% <100.00%> (-0.08%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5978a92...8e128fa. Read the comment docs.