fix: ssrBuild is optional, avoid breaking VitePress

docs/config/index.md

@@ -80,7 +80,7 @@ export default defineConfig(({ command, mode, ssrBuild }) => {
 
 It is important to note that in Vite's API the `command` value is `serve` during dev (in the cli `vite`, `vite dev`, and `vite serve` are aliases), and `build` when building for production (`vite build`).
 
-Only `ssrBuild` is included instead of a more general `ssr` flag because, during dev, the config is shared by the single server handling SSR and non-SSR requests.
+`ssrBuild` is experimental. It is only available during build instead of a more general `ssr` flag because, during dev, the config is shared by the single server handling SSR and non-SSR requests. The value could be `undefined` for tools that don't have separate commands for the browser and SSR build, so use explicit comparison against `true` and `false`.
 
 ## Async Config
 

packages/vite/src/node/config.ts

@@ -66,7 +66,10 @@ export type { RenderBuiltAssetUrl } from './build'
 export interface ConfigEnv {
   command: 'build' | 'serve'
   mode: string
-  ssrBuild: boolean
+  /**
+   * @experimental
+   */
+  ssrBuild?: boolean
 }
 
 /**