Make panel_custom backwards compatible

[balloob]

Breaking change

Proposed change

#36464 accidentally broke the config option where if you specify no urls at all, we fall back to using the name to find an HTML file. This PR restores it.

Also adds a guard so we don't blow up when we see duplicate names for HTML panels. Fixes #36841

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes #
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