luguo

Not exactly sure what is your question. I think the following example may help you to understand "空" in macro.

%let a=%str( );
%let b=;
%let c=C;

%put >>>&a<<<;
%put >>>&b<<<;
%put >>>&c<<<;

You should mask special characters and mnemonic operators.
%NRBQUOTE is the most useful.
%if %NRBQUOTE(&dx1.) ne %then ....

Ref:SAS 9.2 Macro Language: Reference    Macro Quoting

%macro tt(a=);
   %if &a eq   %then  %put  parameter a is null;
   %else %put  parameter a is "&a";
%mend;

%tt(a=)
%tt(a=a)

如果是这样呢？

%let a=%str();
%let b=;


%put >>>&a<<<;
%put >>>&b<<<;