Avoid PHP warnings about undefined indexes when sanitizing malformed PEMs

Author: crocodele

src/Library/KeyManagement/KeyConverter/KeyConverter.php

@@ -399,9 +399,9 @@ private static function sanitizePEM(string &$pem): void
         preg_match_all('#(-.*-)#', $pem, $matches, PREG_PATTERN_ORDER);
         $ciphertext = preg_replace('#-.*-|\r|\n| #', '', $pem);
 
-        $pem = $matches[0][0] . PHP_EOL;
+        $pem = ($matches[0][0] ?? '') . PHP_EOL;
         $pem .= chunk_split($ciphertext ?? '', 64, PHP_EOL);
-        $pem .= $matches[0][1] . PHP_EOL;
+        $pem .= ($matches[0][1] ?? '') . PHP_EOL;
     }
 
     /**