[css-variables][css-conditional] allowing a white space only variable value

I assume we want:

   @supports (var-a: ) { }

to succeed, since "var-a: " is a valid declaration.  However, 
css-conditional-3 has:

   supports_declaration_condition
     : '(' S* declaration ')'
     ;

with "declaration" linking to CSS 2.1, where it is defined as:

   declaration : property S* ':' S* value;
   value       : [ any | block | ATKEYWORD S* ]+;
   any         : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
                 | DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
                 | DASHMATCH | ':' | FUNCTION S* [any|unused]* ')'
                 | '(' S* [any|unused]* ')' | '[' S* [any|unused]* ']'
                 ] S*;

This doesn't allow for a property value that consists only of white space.

On the other hand, 
http://dev.w3.org/csswg/css-syntax/#consume-a-declaration seems to allow 
a declaration to be parsed that has no tokens at all after the ':'.  I 
guess that's OK as css-variables then defines that no tokens after the 
':' is invalid?