Change alt text to empty string for avatars (#21875)

Co-authored-by: Renaud Chaput <renchap@gmail.com>

app/javascript/mastodon/components/__tests__/__snapshots__/avatar-test.jsx.snap

@@ -13,7 +13,7 @@ exports[`<Avatar /> Autoplay renders a animated avatar 1`] = `
   }
 >
   <img
-    alt="alice"
+    alt=""
     src="/animated/alice.gif"
   />
 </div>
@@ -32,7 +32,7 @@ exports[`<Avatar /> Still renders a still avatar 1`] = `
   }
 >
   <img
-    alt="alice"
+    alt=""
     src="/static/alice.jpg"
   />
 </div>

app/javascript/mastodon/components/avatar.tsx

@@ -42,7 +42,7 @@ export const Avatar: React.FC<Props> = ({
       onMouseLeave={handleMouseLeave}
       style={style}
     >
-      {src && <img src={src} alt={account?.get('acct')} />}
+      {src && <img src={src} alt='' />}
     </div>
   );
 };