fix: Adding request-id header in case of empty header value in request

[shubhammxv]

Description

1. Adding request-id header in case header value comes empty in original request

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)

[Baoyuantop]

Hi @shubhammxv, could you explain in detail why this modification was necessary? What is your specific scenario?

[shubhammxv]

Hi @Baoyuantop

In all the scenarios, we return x-request-id in response headers when include_in_response is true except when it comes as empty.
When it is passed as empty, then no header comes back in response (empty value header gets omitted) and it gives perception as if plugin is not working.

Also, the plugin's main functionality is to track the requests, But empty value will not really help track it further.