docs(fields.mdx): fix slugifyTitle example issues

[james-beston]

Removed a redundant const on return slug and added a fallback empty string to satisfy value type.

[github-actions]

Pull Request titles must follow the Conventional Commits specification and have valid scopes.

Unknown scope "fields.mdx" found in pull request title "docs(fields.mdx): fix slugifyTitle example issues". Scope must match one of: cpa, claude, codemod, db-*, db-d1-sqlite, db-mongodb, db-postgres, db-vercel-postgres, db-sqlite, db-d1-sqlite, drizzle, email-*, email-nodemailer, email-resend, eslint, evals, graphql, kv, kv-redis, live-preview, live-preview-react, live-preview-vue, next, payload-cloud, plugin-cloud, plugin-cloud-storage, plugin-ecommerce, plugin-form-builder, plugin-import-export, plugin-mcp, plugin-multi-tenant, plugin-nested-docs, plugin-redirects, plugin-search, plugin-sentry, plugin-seo, plugin-stripe, richtext-*, richtext-lexical, sdk, skills, storage-*, storage-azure, storage-gcs, storage-r2, storage-vercel-blob, storage-s3, tanstack-start, translations, ui, templates, examples(/(\w|-)+)?, deps.

feat(ui): add Button component
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