Match Go TTL Display: return empty string when unit is Empty

Go checks `t.Unit == Empty` separately and returns "" for TTLs
with nonzero count but Empty unit. Rust only checked is_empty()
(count==0 && unit==0), so count>0 with unit=0 would format as
"5 " instead of "".

seaweed-volume/src/storage/needle/ttl.rs

@@ -187,7 +187,7 @@ fn unit_to_char(unit: u8) -> char {
 
 impl fmt::Display for TTL {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if self.is_empty() {
+        if self.count == 0 || self.unit == TTL_UNIT_EMPTY {
             return write!(f, "");
         }
         write!(f, "{}{}", self.count, unit_to_char(self.unit))