Improve java kotlin interop

[fo2rist]

• Check for nulls explicitly for Firebase data until we have Kotlin-ready
• Make ExperimentConfigFactory extension functions better discoverable
• Add missing test for ExperimentConfig

[codecov]

Codecov Report

Merging #19 into master will increase coverage by 1.57%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #19      +/-   ##
============================================
+ Coverage     95.27%   96.85%   +1.57%     
- Complexity       57       63       +6     
============================================
  Files            20       20              
  Lines           127      127              
  Branches         10       11       +1     
============================================
+ Hits            121      123       +2     
+ Misses            3        2       -1     
+ Partials          3        2       -1     

Impacted Files	Coverage Δ	Complexity Δ
...o2rist/cadabra/firebase/ExperimentConfigFactory.kt	100.00% <100.00%> (+33.33%)	6.00 <0.00> (+6.00)
...main/java/com/fo2rist/cadabra/ExperimentsConfig.kt	100.00% <0.00%> (+20.00%)	3.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c9f133c...69bf9a4. Read the comment docs.