implement test

Author: tomoikey

src/rule/non_empty/non_empty_set.rs

@@ -80,6 +80,13 @@ mod test {
     use crate::rule::NonEmptyHashSet;
     use std::collections::HashSet;
 
+    #[test]
+    fn test_err() -> anyhow::Result<()> {
+        let set_result = NonEmptyHashSet::new(HashSet::<u8>::new());
+        assert!(set_result.is_err());
+        Ok(())
+    }
+
     #[test]
     fn test_len() -> anyhow::Result<()> {
         let mut set = HashSet::new();