fix: protect null frontmatter.tags in TaskFile.ts

[evolutioned]

Types of changes

Changes visible to users:

• [ ] New feature (prefix: feat - non-breaking change which adds functionality)
  • Issue/discussion:
  • WARNING: If the link is absent, the PR may be closed without review, due to the workload that such reviews usually require.
• [ ] Breaking change (prefix: feat!! or fix!! - fix or feature that would cause existing functionality to not work as expected)
  • Issue/discussion:
• [x] Bug fix (prefix: fix - non-breaking change which fixes an issue)
• [ ] Documentation (prefix: docs - improvements to any documentation content for users)
• [ ] Sample vault (prefix: vault - improvements to the Tasks-Demo sample vault)
• [ ] Contributing Guidelines (prefix: contrib - any improvements to documentation content for contributors - see Contributing to Tasks)

Internal changes:

• [ ] Refactor (prefix: refactor - non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)
• [ ] Tests (prefix: test - additions and improvements to unit tests and the smoke tests)
• [ ] Infrastructure (prefix: chore - examples include GitHub Actions, issue templates)

Description

Motivation and Context

How has this been tested?

Screenshots (if appropriate)

Checklist

• [ ] My code follows the code style of this project and passes yarn run lint.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] My change has adequate Unit Test coverage.

Terms

• [ ] My contribution follows this project's contributing guide
• [ ] I agree to follow this project's Code of Conduct