[PDI-11194] - fixed bug with DataGrid transformation of empty strings as null

1. DataGridMeta.java : Changed loading behavior while reading empty string data from xml file
2. ValueMetaBase.java : Fixed convert algorythm of empty strings 

@mattyb149 @dkincade 
This should have a null check around it, and throw a KettleValueException if convertMeta is null. Also a unit test to show this new behavior, thanks!