Fix a bug when theme.font has empty family value

[1v9]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

When theme.font has empty family value, the font list will contain empty value, thus resulting in the final string format being incorrect, and there are several situations.

What is the new behavior?

Users could live font family value empty.

Does this PR introduce a breaking change?

• Yes.
• No.