breeze: fix localhost access regression in dual-stack API server startup

[kevinhongzl]

related: #67320

As mentioned in this comment, I can't open the Airflow UI after starting Breeze as well. This issue occurs because Uvicorn requires the host to be set to an empty string in order to bind correctly to both IPv4 and IPv6 interfaces.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.