Fix nested blueprint url_prefix

This fixes the case where the blueprint is registered with a
url_prefix but any child blueprints have no prefixes.

src/flask/blueprints.py

@@ -354,6 +354,8 @@ class Blueprint(Scaffold):
                 bp_options["url_prefix"] = (
                     state.url_prefix.rstrip("/") + "/" + bp_url_prefix.lstrip("/")
                 )
+            else:
+                bp_options["url_prefix"] = state.url_prefix
 
             bp_options["name_prefix"] = options.get("name_prefix", "") + self.name + "."
             blueprint.register(app, bp_options)