[NIP-96] `validateFileUploadResponse` considers an empty `message` string as an invalid response

[eyemono-moe]

When the message in the response object is an empty string, validateFileUploadResponse treats it as an invalid response. This is because !response.message is used in the if statement.

https://github.com/nbd-wtf/nostr-tools/blob/bf0c4d4988a31670e788b200e787c51d20900868/nip96.ts#L272-L274

Some file storage servers (e.g., https://github.com/quentintaranpino/nostrcheck-server) return an empty message, which results in responses from such file storage servers being considered invalid. Is this the intended behavior?

reproduction

import { validateFileUploadResponse } from "nostr-tools/nip96";

const valid = validateFileUploadResponse({
  status: "success",
  message: "not empty",
  nip94_event: {
    tags: [
      ["url", "https://example.com/test.webp"],
      ["ox", "..."],
    ],
  },
});
console.log(valid); // -> true

const invalid = validateFileUploadResponse({
  status: "success",
  message: "", // empty
  nip94_event: {
    tags: [
      ["url", "https://example.com/test.webp"],
      ["ox", "..."],
    ],
  },
});
console.log(invalid); // -> false

[AsaiToshiya]

Probably, I think it seems necessary to compare to undefined not as falsy value.