TranslatablePropertyTransformer should return empty string

[Douglas Hammond]

admin-ui/src/lib/Form/DataTransformer/TranslatablePropertyTransformer.php

Line 31 in d5c026d

 return null; 

As translation lists ate lang => string values returning null is invalid and causes null to be serialized when persisting to the database instead of strings.

If null is suposed to be a valid value then https://github.com/ibexa/core/blob/11067400de1ceef4a679e7743dbac84fa1b172cb/src/lib/Repository/Mapper/ContentDomainMapper.php#L817 should be corrected to handle this.