fix: don't report empty environment variables as missing

.changeset/env-empty-string.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't report empty environment variables as missing

packages/kit/src/exports/internal/env.js

@@ -27,7 +27,7 @@ export function validate(variables, value, name, issues) {
 	const validator = config.schema;
 
 	if (!validator) {
-		if (!value) issues[name] = [MISSING];
+		if (value === undefined) issues[name] = [MISSING];
 		return value;
 	}
 

packages/kit/src/exports/internal/env.spec.js

@@ -0,0 +1,19 @@
+import { assert, test } from 'vitest';
+import { validate } from './env.js';
+
+test('reports a variable without a validator as missing when it is undefined', () => {
+	/** @type {Record<string, import('@standard-schema/spec').StandardSchemaV1.Issue[]>} */
+	const issues = {};
+	validate({}, undefined, 'FOO', issues);
+
+	assert.ok(issues.FOO);
+});
+
+test('accepts an empty string for a variable without a validator', () => {
+	/** @type {Record<string, import('@standard-schema/spec').StandardSchemaV1.Issue[]>} */
+	const issues = {};
+	const value = validate({}, '', 'FOO', issues);
+
+	assert.equal(value, '');
+	assert.deepEqual(issues, {});
+});

packages/kit/src/exports/public.d.ts

@@ -2457,7 +2457,7 @@ export interface EnvVarConfig<T> {
 	 * The validator can output any value — not necessarily a string — but public, non-static values must be
 	 * serializable by [devalue](https://github.com/sveltejs/devalue) so that they can be sent to the browser.
 	 *
-	 * If omitted, the value must be a non-empty string.
+	 * If omitted, the value must be set, but may be an empty string.
 	 */
 	schema?: StandardSchemaV1<string | undefined, T>;
 	/**

packages/kit/types/index.d.ts

@@ -2414,7 +2414,7 @@ declare module '@sveltejs/kit' {
 		 * The validator can output any value — not necessarily a string — but public, non-static values must be
 		 * serializable by [devalue](https://github.com/sveltejs/devalue) so that they can be sent to the browser.
 		 *
-		 * If omitted, the value must be a non-empty string.
+		 * If omitted, the value must be set, but may be an empty string.
 		 */
 		schema?: StandardSchemaV1<string | undefined, T>;
 		/**