CAMT.053.001.08 without optional CreDtTm in statements throughs exception

[gregor-tb]

Hello,

parsing some CAMT.053.001.08 Files crashes with "Cannot decode empty string as a date"

            $statement = new Camt053DTO\Statement(
                (string) $xmlStatement->Id,
                $this->dateDecoder->decode((string) $xmlStatement->CreDtTm),
                $this->getAccount($xmlStatement)
            );

The reason is CreDtTm is empty, similar to issue #175

But as far as I checked the reference, this is an 0..1 field in this 8th version, so it should be fine (and maybe take the CreDtTm from the group header idk?).

Thanks!