fix non string value check

src/Application/Model/Credential/PublicKeyCredential.php

@@ -175,6 +175,6 @@ class PublicKeyCredential extends BaseModel
             );
         $oxid = $qb->execute()->fetchOne();
 
-        return strlen($oxid) ? $oxid : null;
+        return is_string($oxid) && strlen($oxid) ? $oxid : null;
     }
 }

src/Application/Model/Credential/PublicKeyCredentialList.php

@@ -66,7 +66,7 @@ class PublicKeyCredentialList extends ListModel implements PublicKeyCredentialSo
             );
         $credential = $qb->execute()->fetchOne();
 
-        if (!strlen($credential)) {
+        if (!is_string($credential) || !strlen($credential)) {
             return null;
         }