Fixed the pom packaging bug

[mockitoguy]

Fixes #847 - users can configure pom in the build.file so our logic cannot blindly add XML nodes - we need to check for existence of nodes.

This is a wider problem - this PR fixes one instance - see the TODO in the code.

Happy New Year everybody :-)

[codecov]

Codecov Report

Merging #854 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #854      +/-   ##
============================================
+ Coverage     76.85%   76.85%   +<.01%     
- Complexity     1229     1230       +1     
============================================
  Files           219      219              
  Lines          4467     4468       +1     
  Branches        334      335       +1     
============================================
+ Hits           3433     3434       +1     
  Misses          922      922              
  Partials        112      112

Impacted Files	Coverage Δ	Complexity Δ
...rg/shipkit/internal/gradle/util/PomCustomizer.java	75.8% <100%> (+0.39%)	11 <0> (+1)	⬆️

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[mockitoguy]

Thank you for review!