fixed bug with DataGrid transformation of empty strings as null
[PDI-11194] fixed the cause of integration test failure after the previous
merging
[PDI-11194] fixed case of summation of number values in number form to number values in empty string form
Corrected checkstyle violations
Merge pull request #839 from andrey-markelov/SP-1624

[SP-1624] fixed bug with DataGrid transformation of empty strings