fix: remove duplicate empty-string alternative in continuation_token regex

The ListStoresRequest.continuation_token validation pattern had a
redundant '^$|' prefix, making it '^$|^$|^[A-Za-z0-9-_]+={0,2}$'
instead of '^$|^[A-Za-z0-9-_]+={0,2}$' like all other continuation_token
fields in the file.

Author: Siddhant-K-code

openfga/v1/openfga_service.proto

@@ -1835,7 +1835,7 @@ message ListStoresRequest {
   string continuation_token = 2 [
     json_name = "continuation_token",
     (validate.rules).string.max_bytes = 5120,
-    (validate.rules).string.pattern = "^$|^$|^[A-Za-z0-9-_]+={0,2}$",
+    (validate.rules).string.pattern = "^$|^[A-Za-z0-9-_]+={0,2}$",
     (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {example: "\"eyJwayI6IkxBVEVTVF9OU0NPTkZJR19hdXRoMHN0b3JlIiwic2siOiIxem1qbXF3MWZLZExTcUoyN01MdTdqTjh0cWgifQ==\""}
   ];