Fix handling of nonexistent MLTools in API

Codecov Report

Merging #273 into master will increase coverage by 0.06%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   79.31%   79.37%   +0.06%     
==========================================
  Files         160      161       +1     
  Lines        9532     9561      +29     
==========================================
+ Hits         7560     7589      +29     
  Misses       1972     1972

Impacted Files	Coverage Δ
arkindex/project/tests/test_arrayfield.py	100% <ø> (ø)
arkindex/project/serializer_fields.py	100% <100%> (ø)
arkindex/project/tests/test_mltoolfield.py	100% <100%> (ø)

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merged

mentioned in commit 1dc6bbe9