Fix infinite loading instead of soft 404 for non-existing remote accounts (#21303)

Fixes #21278, #21021

app/javascript/mastodon/features/account_timeline/index.js

@@ -26,7 +26,13 @@ const emptyList = ImmutableList();
 const mapStateToProps = (state, { params: { acct, id, tagged }, withReplies = false }) => {
   const accountId = id || state.getIn(['accounts_map', normalizeForLookup(acct)]);
 
-  if (!accountId) {
+  if (accountId === null) {
+    return {
+      isLoading: false,
+      isAccount: false,
+      statusIds: emptyList,
+    };
+  } else if (!accountId) {
     return {
       isLoading: true,
       statusIds: emptyList,

app/javascript/mastodon/reducers/accounts_map.js

 import { ACCOUNT_IMPORT, ACCOUNTS_IMPORT } from '../actions/importer';
+import { ACCOUNT_LOOKUP_FAIL } from '../actions/accounts';
 import { Map as ImmutableMap } from 'immutable';
 
 export const normalizeForLookup = str => str.toLowerCase();
@@ -7,6 +8,8 @@ const initialState = ImmutableMap();
 
 export default function accountsMap(state = initialState, action) {
   switch(action.type) {
+  case ACCOUNT_LOOKUP_FAIL:
+    return action.error?.response?.status === 404 ? state.set(normalizeForLookup(action.acct), null) : state;
   case ACCOUNT_IMPORT:
     return state.set(normalizeForLookup(action.account.acct), action.account.id);
   case ACCOUNTS_IMPORT: