Fix npe in validation and transformation

[salander85]

Summary

The sync breaks when the referenced keys of a resource are null / "null".
JIRA: https://commercetools.atlassian.net/browse/DEVX-274

Description

After the migration to java-sdk-v2 the graphQl requests changed from a SphereRequest with deserialization of JavaObjects via Jackson to this implementation using object-mapper to map the data-object of GraphQlResult.

The fix in ProductBatchValidator was done, because the given resource drafts might not be transformed by the tool and therefore could have invalid values.

Relevant Issues

#1101

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0f91d9a) 97.58% compared to head (aa29202) 97.63%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1102      +/-   ##
============================================
+ Coverage     97.58%   97.63%   +0.04%     
- Complexity     2655     2662       +7     
============================================
  Files           229      229              
  Lines          8455     8459       +4     
  Branches        510      513       +3     
============================================
+ Hits           8251     8259       +8     
+ Misses          131      128       -3     
+ Partials         73       72       -1     

Files	Coverage Δ
...tools/sync/commons/helpers/BaseBatchValidator.java	100.00% <ø> (+13.63%)	⬆️
...s/sync/products/helpers/ProductBatchValidator.java	100.00% <100.00%> (ø)
...roducttypes/helpers/ProductTypeBatchValidator.java	100.00% <100.00%> (ø)
.../commercetools/sync/services/impl/BaseService.java	96.82% <100.00%> (+0.07%)	⬆️
...s/sync/services/impl/BaseTransformServiceImpl.java	87.50% <100.00%> (+1.65%)	⬆️
...tools/sync/commons/utils/CustomValueConverter.java	77.77% <75.00%> (-2.23%)	⬇️

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