[Bug] When using `👍` emoji icon. docfx failed to embed relating font.

[filzrev]

Describe the bug
When using thumbs up (👍) emoji on markdown document.
PDF file that generated by GitHub Action's on ubuntu-latest runner failed to embed fonts.
And T3_font is displayed on Adobe Acrobat reader's property page.

To Reproduce
Steps to reproduce the behavior:

1. Download PDF file from https://dotnet.github.io/docfx/seed/articles/seed.pdf
2. Open PDF file with Adobe Acrobat reader.
3. Open [Property]-[Fonts] page and T3 Font is displayed. (It's not embedded on PDF and result depend on environment)

Expected behavior
Fonts should be embedded on PDF. and T3 Font should not be displayed on PDF page.

Additional context
When PDF file is opened on Windows WSL-Ubuntu environment.
thumbs up (👍) emoji is rendered as 􏿮 character. and outputted to JSON as \u0000.
Snapshot tests result JSON cause diffs and emoji is rendered as \u0000.

It can be resolved by manually installing fonts-noto-color-emoji font on WSL environment.

sudo apt install fonts-noto-color-emoji