[BUGFIX] Skip bearer auth flow when Authorization header is absent

authenticateBearerToken() destructures the scheme and token from the
Authorization header. The previous guard returned early only when a
scheme was present but did not equal "bearer". For requests without
an Authorization header (or with an unparseable header that yields no
scheme), $scheme is null, the guard did not trigger, and the function
fell through to TokenRepository::findBackendUserIdByToken(null) which
raised a TypeError because the argument is typed as string.

Invert the guard so it also early-returns when the scheme is not a
string. Smallest possible change: keeps the same single-statement
structure and the same is_string() helper that was already used.

Author: uluzox

Classes/Authentication/HttpBackendUserAuthentication.php

@@ -115,7 +115,7 @@ protected function authenticateBearerToken(ServerRequestInterface $request): voi
 
         [$scheme, $token] = GeneralUtility::trimExplode(' ', $authorizationHeader, true);
 
-        if (is_string($scheme) && strtolower($scheme) !== 'bearer') {
+        if (!is_string($scheme) || strtolower($scheme) !== 'bearer') {
             return;
         }