Update lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php

Co-authored-by: Carl Schwan <carl@carlschwan.eu>
Signed-off-by: Micke Nordin <mickenordin@users.noreply.github.com>

Author: mickenordin

lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php

@@ -294,7 +294,7 @@ private function reconstructTargetUri(): string {
 	 * lowercased name. Derived components (`@*`) are produced inside
 	 * {@see SignatureBase}; we only collect plain fields here.
 	 *
-	 * @return array<string, string>
+	 * @return array<non-empty-string, non-empty-string>
 	 */
 	private function collectHeaders(): array {
 		$out = [];