I agree with @s_lassen ; functions called by %SYSFUNC need at least one argument, even if the value of the argument is null. The argument cannot be empty.
In the DATA step language, all of the below error, because the function call has no arguments:
data _null_ ;
x=trimn() ;
x=upcase() ;
x=strip() ;
x=compress() ;
run ;
If you want to pass a null value to a function in the DATA step language, you can do it like:
data _null_ ;
x=trimn("") ;
x=upcase("") ;
x=strip("") ;
x=compress("") ;
run ;
In the macro language calling a function via %SYSFUNC, all of the below error, because the function call has no arguments:
%put %sysfunc(trimn()) ;
%put %sysfunc(upcase()) ;
%put %sysfunc(strip()) ;
%put %sysfunc(compress()) ;
If you want to pass a null value to a macro language function called via %SYSFUNC, you can do it like:
%put %sysfunc(trimn(%str())) ;
%put %sysfunc(upcase(%str())) ;
%put %sysfunc(strip(%str())) ;
%put %sysfunc(compress(%str())) ;
I see the above as consistent.
As for the %SYSFUNC docs, I'm not sure how to understand the bit you quote: "In particular, an empty argument position will not generate a NULL argument, but a zero length argument." I'm not sure what they are meaning for the difference between a null argument and a zero length argument.
Note that the error message caused by an empty function argument is coming from %SYSFUNC, not from the called function. And if you read the documentation for %SYSFUNC, it defines the arguments to the function as "one or more arguments used by function." So basically, %SYSFUNC must have something to pass to the function, it does not allow zero arguments.
For functions like COMPRESS, which accept an optional comma as a delimiter for multiple arguments, you can even pass just the comma:
47 %put >>%sysfunc(compress(,))<< ;
>><<
In that case, I think %SYSFUNC is happy because it sees the comma so it has something it can pass to compress. Then compress is happy, because it gets the comma, and says "oh, I got passed two null arguments, and I'm good with that."
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