v.util: make util.smart_quote use its output string builder directly

vlib/strings/builder.v

@@ -45,7 +45,7 @@ pub fn (b &Builder) byte_at(n int) byte {
 // write appends the string `s` to the buffer
 [inline]
 pub fn (mut b Builder) write_string(s string) {
-	if s == '' {
+	if s.len == 0 {
 		return
 	}
 	unsafe { b.push_many(s.str, s.len) }