[ISSUE #6842] Handle missing data in HTTP long-polling responses

[yykaue]

What does this PR do?

This PR fixes the null handling in HttpSyncDataService.doLongPolling().

HTTP long-polling responses without configuration change data are now treated as “no change” instead of causing an exception.

The following response cases are handled:

• The data field is missing.
• The data field is null.
• The root response is null.
• The response body is empty.

Valid data arrays continue to use the existing processing flow.

Closes #6842

@Aias00, could you please help review this PR? Thanks!

[Aias00]

This now treats admin error payloads as successful no-change responses. The admin exception handlers return ShenyuAdminResult.error(...) bodies such as {"code":500,"message":"..."} without setting a non-200 HTTP status, and this branch returns as soon as data is missing/null before checking the result code. In HttpLongPollingTask, only exceptions from doLongPolling trigger the retry/backoff path, so a listener-side error body can now be swallowed as no change and the loop immediately continues. Could we check code first and throw/retry when it is present and not CommonErrorCode.SUCCESSFUL, then only ignore missing/null data for successful responses?