You can NMISS() function if ALL of the variables are character. Or use the CMISS() function if ANY of the variables are character.
If you just want to test if ANY of the variables are missing then do:
if nmiss(of var1 var5 var13) then newvar=.;
If you need to test if ALL of the variables are missing and all of them are numeric then you could use the N() function instead.
if N(of var1 var5 var13)=0 then newvar=.;
But if any are character then you need to know how many there are.
if cmiss(of var1 var5 var13)=3 then newvar=.;