Add option for setting NONE character of Special Value property instead of setting the full absolute value without changing padding character

We need to have an option to set the character used in the NONE Special Value property in a type tree's field. The only way it works now is by setting the exact value to be evaluated to NONE. So, for example, if we are setting the NONE special value of a three-character long and a five-character long text fields to have <NULL> bytes instead of the default spaces and want those values to be evaluated to NONE, we have to set the NONE Special Value of the first field to "<NULL><NULL><NULL>" and the second to "<NULL><NULL><NULL><NULL><NULL>". There doesn't seem to be a way to just say that we want the default NONE byte of this field to be "<NULL>" instead of the default "<SP>" for text fields or zero for numeric fields. Of course this example is a simple one, but in our case we have over 1000 fields that range from 5 characters to over 500 characters, so setting the exact absolute value of all those fields doesn't make sense and will make the type tree extremely hard to maintain in case the copybook needed to be reimported, which happens twice a year anyways in this project. 

Note: This needs to be applied without changing the padding characters. So for text fields, for example, the padding character to still be <SP>, or whatever character it is, and zero for numeric fields, and so on.