py/objstr: Fix b"".hex() so it returns an empty str object.

As per CPython behaviour, `b"".hex()` should return an empty str object
(not an empty bytes object).

Fixes issue micropython#18807.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

py/objstr.c

@@ -1995,7 +1995,7 @@ mp_obj_t mp_obj_bytes_hex(size_t n_args, const mp_obj_t *args, const mp_obj_type
     // Code below assumes non-zero buffer length when computing size with
     // separator, so handle the zero-length case here.
     if (bufinfo.len == 0) {
-        return mp_const_empty_bytes;
+        return make_empty_str_of_type(type);
     }
 
     vstr_t vstr;

tests/basics/builtin_str_hex.py

@@ -3,6 +3,7 @@
     raise SystemExit
 
 for x in (
+    b"",
     b"\x00\x01\x02\x03\x04\x05\x06\x07",
     b"\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
     b"\x7f\x80\xff",