Implement correct output for empty strings

Fixes #988

Author: mgreter

functions.cpp

@@ -940,7 +940,7 @@ namespace Sass {
         error(msg, pstate, backtrace);
       }
       catch (...) { throw; }
-      return new (ctx.mem) String_Constant(pstate, newstr);
+      return new (ctx.mem) String_Quoted(pstate, newstr);
     }
 
     Signature to_upper_case_sig = "to-upper-case($string)";

output.cpp

@@ -134,7 +134,7 @@ namespace Sass {
             String_Constant* valConst = static_cast<String_Constant*>(dec->value());
             string val(valConst->value());
             if (dynamic_cast<String_Quoted*>(valConst)) {
-              if (val.empty()) {
+              if (!valConst->quote_mark() && val.empty()) {
                 bPrintExpression = false;
               }
             }