More flexible handling of AssertJ isNotNull methods

[msridhar]

Fixes #1219

Previously, we would only consider the isNotNull method of org.assertj.core.api.AbstractAssert to be valid. Now, we allow isNotNull of any class that implements the org.assertj.core.api.Assert interface, which handles the case from #1219 and more.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.31%. Comparing base (6cfccfd) to head (116f1a9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1221   +/-   ##
=========================================
  Coverage     88.30%   88.31%           
- Complexity     2353     2356    +3     
=========================================
  Files            89       89           
  Lines          7662     7668    +6     
  Branches       1535     1536    +1     
=========================================
+ Hits           6766     6772    +6     
  Misses          450      450           
  Partials        446      446           

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