Fixed bug with calling clean on None values

[jackton1]

Fixes: #453

[codecov]

Codecov Report

Merging #456 (e034b4e) into main (9a97352) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #456   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          283       283           
=========================================
  Hits           283       283           

Impacted Files	Coverage Δ
model_clone/mixins/clone.py	100.00% <100.00%> (ø)

---

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 9a97352...e034b4e. Read the comment docs.