Empty strings are not valid relative JSON pointers.

(This is unlike non-relative pointers, where empty strings are indeed
valid.)

Refs: json-schema-org/JSON-Schema-Test-Suite#605

jsonschema/_format.py

@@ -454,6 +454,9 @@ def is_relative_json_pointer(instance: object) -> bool:
         # https://tools.ietf.org/html/draft-handrews-relative-json-pointer-01#section-3
         if not isinstance(instance, str):
             return True
+        if not instance:
+            return False
+
         non_negative_integer, rest = [], ""
         for i, character in enumerate(instance):
             if character.isdigit():