[Bug] PDF Header/Footer template path parsing

[HollyAssasin]

When specifiying a relative path to an HTML file containing the footer/header path, it instead puts the path string itself as the header/footer in the PDF.

My file structure:

/Docfx
├── docfx.json
├── pdf_templates/
│   └── header.html
└── _site/

Image of the bug:

To Reproduce
Steps to reproduce the behavior:

1. Create an HTML template
2. Link it in the docfx.json ("pdfHeaderTemplate": "pdf_templates/header.html")
3. Open the PDF and see

Expected behavior
Expected to see the provided Footer/Header template

Context

• OS: Windows
• Docfx version: Nugget 2.77.0

[filzrev]

The reported problem appears to be the same as #9990.
It's fixed by #10099, but not included in the latest released (v2.77.0) version.