don't write null values to pom

[gabrielittner]

closes #76

[codecov]

Codecov Report

Merging #89 into master will decrease coverage by 0.31%.
The diff coverage is 71.42%.

@@             Coverage Diff              @@
##             master      #89      +/-   ##
============================================
- Coverage     58.33%   58.02%   -0.32%     
- Complexity       45       46       +1     
============================================
  Files             8        8              
  Lines           288      293       +5     
  Branches         34       35       +1     
============================================
+ Hits            168      170       +2     
- Misses           96       98       +2     
- Partials         24       25       +1

Impacted Files	Coverage Δ	Complexity Δ
...vanniktech/maven/publish/MavenPublishConfigurer.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...in/com/vanniktech/maven/publish/MavenPublishPom.kt	95% <100%> (+0.26%)	17 <2> (+1)	⬆️
...vanniktech/maven/publish/MavenPublishPlugin.groovy	63.71% <50%> (-1.15%)	8 <0> (ø)

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