fix(batch-requests): the number of sub-responses does not match that of sub-requests

[hachi029]

Description

when there is any timeout sub request, the number of sub responses does not match the number of sub requests。

for example:

the batch-request with 2 sub requests
{"headers":{},"pipeline":[{"path":"/ok","method":"GET"},{"path":"/timeout","method":"GET"}]}

there are 3 sub responses in the batch-response, with an extra empty json object '{}'
[{"status":200,"body":"ok","reason":"OK","headers":{"Connection":"keep-alive"}},{"status":504,"reason":"upstream timeout"},{}]

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)