clarify that base path can be the empty string

.changeset/old-cars-switch.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Tweak error message for non-validating base paths

documentation/docs/14-configuration.md

@@ -224,7 +224,7 @@ export default config;
 An object containing zero or more of the following `string` values:
 
 - `assets` — an absolute path that your app's files are served from. This is useful if your files are served from a storage bucket of some kind
-- `base` — a root-relative path that must start, but not end with `/` (e.g. `/base-path`). This specifies where your app is served from and allows the app to live on a non-root path
+- `base` — a root-relative path that must start, but not end with `/` (e.g. `/base-path`), unless it is the empty string. This specifies where your app is served from and allows the app to live on a non-root path
 
 ### prerender
 

packages/kit/src/core/config/options.js

@@ -161,7 +161,7 @@ const options = object(
 
 					if (input !== '' && (input.endsWith('/') || !input.startsWith('/'))) {
 						throw new Error(
-							`${keypath} option must be a root-relative path that starts but doesn't end with '/'. See https://kit.svelte.dev/docs/configuration#paths`
+							`${keypath} option must either be the empty string, or a root-relative path that starts but doesn't end with '/'. See https://kit.svelte.dev/docs/configuration#paths`
 						);
 					}