The following script will synthesize document categories for invalid Final_DocumentCategory_Id values on documents.

-- Some documents are linked to categories that no longer exist.
-- Create dummy document categories based on these values.
INSERT INTO
  __M_Documents_Digital_Categories (
    Id,
    Final_ReferenceCode,
    Final_Subject
)
SELECT DISTINCT
    Final_DocumentCategory_Id,
    Final_DocumentCategory_Id, 
    Final_DocumentCategory_Id
FROM
    __M_Documents_Digital_Files
WHERE 1=1
    AND Final_DocumentCategory_Id != ''
    AND Final_DocumentCategory_Id NOT IN (SELECT Id FROM __M_Documents_Digital_Categories)