NullPointerException Multi-level inheritance fixes #41

[DarrenC]

This Pull Request is based on the solution given by @stroisi in issue #41.

It also adds some Unit Tests based on JUnit 5

[codecov[bot]]

Codecov Report

Merging #45 into master will increase coverage by 67.79%. The diff coverage is 92.85%.

@@             Coverage Diff             @@
##           master      #45       +/-   ##
===========================================
+ Coverage       0%   67.79%   +67.79%     
===========================================
  Files           9        9               
  Lines         611      621       +10     
  Branches      104      107        +3     
===========================================
+ Hits            0      421      +421     
+ Misses        611      160      -451     
- Partials        0       40       +40

Impacted Files	Coverage Δ
...icksolutions/swagger/plantuml/PlantUMLCodegen.java	75.9% <92.85%> (+75.9%)	:arrow_up:
...icksolutions/swagger/plantuml/vo/ClassDiagram.java	44.82% <0%> (+44.82%)	:arrow_up:
...lutions/swagger/plantuml/vo/MethodDefinitions.java	61.53% <0%> (+61.53%)	:arrow_up:
...ksolutions/swagger/plantuml/PlantUMLGenerator.java	61.53% <0%> (+61.53%)	:arrow_up:
...icksolutions/swagger/plantuml/vo/ClassMembers.java	66.66% <0%> (+66.66%)	:arrow_up:
...olutions/swagger/plantuml/vo/InterfaceDiagram.java	66.66% <0%> (+66.66%)	:arrow_up:
...cksolutions/swagger/plantuml/vo/ClassRelation.java	88% <0%> (+88%)	:arrow_up:

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