Invalid arguments passed to implode()

Hello,

I have noticed the following PHP warning in debug.log:
[03-Sep-2020 08:48:27 UTC] PHP Warning: implode(): Invalid arguments passed in wp-content/plugins/post-smtp/Postman/PostmanLogFields.php on line 63

To resolve this, I add one check if json_decode() return an array before attempt to do implode, so replaced line 63:

return implode( ',', json_decode( $json, true ) );

with this:

            $json_array = json_decode( $json, true );
            if ( is_array($json_array) ) {
                return implode( ',', $json_array );
            }

Cheers,
Aleksandar