internal/httpcli: check if resp is nil before attempting to log fields
Created by: bobheadxi
Seeing in main that the response might be nil - https://buildkite.com/sourcegraph/sourcegraph/builds/170045#0182efc0-4e36-4231-81ed-57db47c6fdcc - presumably in the error case. Bug introduced in https://github.com/sourcegraph/sourcegraph/pull/40963
Test plan
this branch runs backend integration tests, should pass