fix: return error when format is empty in parse

The format parsed in from_str maybe empty(eg. '%'). As a result,
cur_item may be None.

src/efmt/format.rs

@@ -156,7 +156,10 @@ impl Format {
         // Previous index of interest in the string
         let mut prev_idx = 0;
         let mut cur_item_idx = 0;
-        let mut cur_item = self.items[cur_item_idx].unwrap();
+        let mut cur_item =  match self.items[cur_item_idx] {
+            Some(item) => item,
+            None => return Err(Errors::ParseError(ParsingErrors::UnknownFormat)),
+        };
         let mut cur_token = cur_item.token;
         let mut prev_item = cur_item;
         let mut prev_token;