Backport of api-gateway: Fix nil pointer exception panic into release/1.2.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #2487 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
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The person who merged in the original PR is:
@nathancoleman
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul-k8s/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Changes proposed in this PR:

• Fix nil pointer exception thrown whenever the user uses an httpfilter that does not have RequestHeaderModifier set.

How I've tested this PR:

• Created unit test that replicated the error and saw that it is resolved with this fix

How I expect reviewers to test this PR:

• Unit tests pass

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

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Overview of commits

• 4070a3c - ddda965 - 5f505ab - 6966207 - b46205a