fix: prevent infinite loop when fetching a list of results

When fetching a list of results from cloudflare APIs (e.g. when fetching a list of keys in a kv namespace), the api returns a `cursor` that a consumer should use to get the next 'page' of results. It appears this cursor can also be a blank string (while we'd only account for it to be `undefined`). By only accounting for it to be `undefined`, we were infinitely looping through the same page of results and never terminating. This PR fixes it by letting it be a blank string (and `null`, for good measure)

Author: threepointone

packages/wrangler/src/cfetch/index.ts

@@ -106,5 +106,6 @@ function throwFetchError(
 }
 
 function hasCursor(result_info: unknown): result_info is { cursor: string } {
-  return (result_info as { cursor: string } | undefined)?.cursor !== undefined;
+  const cursor = (result_info as { cursor: string } | undefined)?.cursor;
+  return cursor !== undefined && cursor !== null && cursor !== "";
 }