allow null @site

Author: cruzdanilo

src/theme.less

@@ -16,7 +16,7 @@
 @import "@{themesFolder}/default/globals/site.variables";
 
 /* Packaged site.variables */
-@import "@{themesFolder}/@{site}/globals/site.variables";
+@import (optional) "@{themesFolder}/@{site}/globals/site.variables";
 
 /* Component's site.variables */
 @import (optional) "@{themesFolder}/@{theme}/globals/site.variables";