fix: allow AdditionalConfigLoader to return void

src/shared/shared.ts

@@ -274,7 +274,8 @@ function resolveAdditionalConfig(
   path: string
 ): AdditionalConfig[] {
   if (additionalConfig === undefined) return []
-  if (typeof additionalConfig === 'function') return additionalConfig(path)
+  if (typeof additionalConfig === 'function')
+    return additionalConfig(path) ?? []
 
   const configs: AdditionalConfig[] = []
   const segments = path.split('/').slice(0, -1) // remove file name

types/shared.d.ts

@@ -192,7 +192,7 @@ export type AdditionalConfigDict<ThemeConfig = any> = Record<
 
 export type AdditionalConfigLoader<ThemeConfig = any> = (
   relativePath: string
-) => AdditionalConfig<ThemeConfig>[]
+) => AdditionalConfig<ThemeConfig>[] | void
 
 // Manually declaring all properties as rollup-plugin-dts
 // is unable to merge augmented module declarations