For the document Control Assistant who was born on 1975/12/25, how many private documents did he/she have?
Document Control Assistant refers to the JobTitle = 'Document Control Assistant'; born on 1975/12/25 refers to BirthDate = '1975-12-25'; private documents indicate that DocumentSummary is null
SELECT COUNT(T2.BusinessEntityID) FROM Document AS T1 INNER JOIN Employee AS T2 ON T1.Owner = T2.BusinessEntityID WHERE T2.JobTitle = 'Document Control Assistant' AND T2.BirthDate = '1975-12-25' AND T1.DocumentSummary IS NULL