fix(v2): Allow the alt for the logo to be empty (#3352)

See
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Img#Attributes

> Setting this attribute to an empty string (alt="") indicates that
> this image is not a key part of the content (it’s decoration or a
> tracking pixel), and that non-visual browsers may omit it from
> rendering.

Since the logo is indeed decorative and redundant with the actual
project name, an empty alt is fine.

packages/docusaurus-theme-classic/src/__tests__/validateThemeConfig.test.js

@@ -93,6 +93,22 @@ describe('themeConfig', () => {
     });
   });
 
+  test('should allow empty alt tags for the logo image in the header', () => {
+    const altTagConfig = {
+      navbar: {
+        logo: {
+          alt: '',
+          src: '/arbitrary-logo.png',
+        },
+        hideOnScroll: false,
+      },
+    };
+    expect(testValidateThemeConfig(altTagConfig)).toEqual({
+      colorMode: DEFAULT_COLOR_MODE_CONFIG,
+      ...altTagConfig,
+    });
+  });
+
   test('should accept valid prism config', () => {
     const prismConfig = {
       prism: {

packages/docusaurus-theme-classic/src/validateThemeConfig.js

@@ -181,7 +181,7 @@ const ThemeConfigSchema = Joi.object({
     items: Joi.array().items(NavbarItemSchema),
     title: Joi.string().allow('', null),
     logo: Joi.object({
-      alt: Joi.string(),
+      alt: Joi.string().allow(''),
       src: Joi.string().required(),
       srcDark: Joi.string(),
       href: Joi.string(),