fix: disable recaptcha when site key is not set

[mderynck]

What this PR does

Although recaptcha verification is disabled by default on the backend for OSS installs, the plugin was still making use of the site key and trying to load recaptcha. As that recaptcha site was removed this no longer works. The updated plugin code will skip recaptcha verification if it does not have a site key set.

Which issue(s) this PR closes

Related to #5449

Checklist

• Unit, integration, and e2e (if applicable) tests updated
• Documentation added (or pr:no public docs PR label added if not required)
• Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will
  show up in the autogenerated release notes.

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