Fix rtrimstr("") always output "" (#3415)

Example
---
**Before this fix**:

```sh
$ jq -n '"foo"|rtrimstr("")'
""
```

**After this fix**:

```sh
$ jq -n '"foo"|rtrimstr("")'
"foo"
```

tests/jq.test

@@ -1512,6 +1512,18 @@ split("")
 ["fo", "foo", "barfoo", "foobarfoo", "foob"]
 ["fo","","bar","bar","b"]
 
+[.[]|ltrimstr("")]
+["a", "xx", ""]
+["a", "xx", ""]
+
+[.[]|rtrimstr("")]
+["a", "xx", ""]
+["a", "xx", ""]
+
+[.[]|trimstr("")]
+["a", "xx", ""]
+["a", "xx", ""]
+
 [(index(","), rindex(",")), indices(",")]
 "a,bc,def,ghij,klmno"
 [1,13,[1,4,8,13]]