Document check digit field issues

[stropitek]

• The location of the check digit (available via result.details) should be changed when there is a < at position 14 (TD1).
• The value of the check digit field should not be null when the check digit is wrong, at least not when it is a number as expected.

It should be simple to reproduce the issue with valid TD1 documents which have the check digit in the optional part, by just changing the check digit number.