Issue with NULL value in nested case

Long story short i need to return a varchar value if stored value is null but eithers bring me a NULL value or varchar value here is the code.

in the table i have some fields null and some with data

CASE WHEN DS.DBIRTH+DS.MBIRTH+DS.YBIRTH = 
'' THEN CONVERT(VARCHAR(10),PT.CLIENTDBIRTHDATE,111)
ELSE 'INPUT DATE OF BIRTH'
CASE WHEN PT.CLIENTBIRTHDATE IS NULL THEN ''
 ELSE '1800-01-01'
 END 
 END AS BFIELD

this brings me something like this

         NULL
        1917/05/02
         NULL
        1923/02/02
        1967/01/05
        NULL

but i need something like this

        01/01/1800
        1917/05/02
        01/01/1800
        1923/02/02
        1967/01/05
        01/01/1800

sorry for the ultra noob question