fixed access to possible null object RequestMessage

[adegwerth]

Description

Avoid access to HttpResponseMessage.RequestHeader if the parameter is null.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[dnfadmin]

All CLA requirements met.