Add test to ensure that nodes can be saved with null complete.

[rtibbles]

Description

Adds a test to ensure that null complete is a valid value on content nodes

[codecov]

Codecov Report

Merging #2764 (229e843) into develop (b56f349) will increase coverage by 0.00%.
The diff coverage is 41.07%.

@@ Coverage Diff @@## develop #2764 +/- ##
========================================
Coverage 84.92% 84.92% ========================================
Files 307 309 +2 Lines 15327 15346 +19 ========================================
+ Hits 13016 13033 +17 - Misses 2311 2313 +2 

Impacted Files	Coverage Δ
...ntcuration/contentcuration/viewsets/contentnode.py	83.73% <0.00%> (ø)
contentcuration/contentcuration/tasks.py	62.58% <24.13%> (ø)
contentcuration/contentcuration/viewsets/user.py	70.56% <33.33%> (ø)
contentcuration/contentcuration/views/admin.py	73.68% <60.00%> (ø)
...uration/contentcuration/tests/test_contentnodes.py	94.60% <66.66%> (-0.39%)	⬇️
contentcuration/contentcuration/decorators.py	56.60% <100.00%> (ø)
contentcuration/contentcuration/viewsets/file.py	72.22% <100.00%> (ø)
...tentcuration/migrations/0129_auto_20210113_1723.py	100.00% <0.00%> (ø)
...entcuration/migrations/0130_merge_20210113_1839.py	100.00% <0.00%> (ø)
... and 4 more

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