Fix nginx webmanifest sub_filter

[leccelecce]

Proposed change

Fix for issues with empty start_url value in webmanifest caused by #17030 - see discussion on that PR for background.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code
• Documentation Update

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• UI changes including text have used i18n keys and have been added to the en locale.
• The code has been formatted using Ruff (ruff format frigate)

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[leccelecce]

I have confirmed this works for a "standard" setup but someone with the ability to test the X-Ingress-Path will need to confirm the functionality works as expected there.