fix(theme-default): fix homepage frontmatter type

Author: meteorlxy

packages/@vuepress/theme-default/src/types/page.ts

@@ -9,8 +9,8 @@ export interface DefaultThemeHomePageFrontmatter {
   home: true
   heroImage?: string
   heroAlt?: string
-  heroText?: string
-  tagline?: string
+  heroText?: string | null
+  tagline?: string | null
   actions?: {
     text: string
     link: string