cac:ExternalReference can't be used without cbc:EmbeddedDocumentBinaryObject

[CasperBE]

cac:AdditionalDocumentReference/cac:Attachment/cac:ExternalReference can be set, but an error is thrown:
XMLWriter::writeAttribute() expects parameter 2 to be string, null given

Proposed change in Attachment.php:

    public function xmlSerialize(Writer $writer): void
    {
        $this->validate();

        if (!empty($this->filePath)) {
            $fileContents = base64_encode(file_get_contents($this->filePath));
            $fileName = basename($this->filePath);
            $mimeType = $this->getFilePathMimeType();

            $writer->write([
               'name'       => Schema::CBC . 'EmbeddedDocumentBinaryObject',
               'value'      => $fileContents,
               'attributes' => [
                   'mimeCode' => $mimeType,
                   'filename' => $fileName,
               ],
           ]);
        } elseif (!empty($this->base64Content)) {
            $fileContents = $this->base64Content;
            $fileName = $this->fileName;
            $mimeType = $this->mimeType;

            $writer->write([
               'name'       => Schema::CBC . 'EmbeddedDocumentBinaryObject',
               'value'      => $fileContents,
               'attributes' => [
                   'mimeCode' => $mimeType,
                   'filename' => $fileName,
               ],
           ]);
        } elseif (!empty($this->externalReference)) {
            $writer->writeElement(
                Schema::CAC . 'ExternalReference',
                [Schema::CBC . 'URI' => $this->externalReference]
            );
        }
    }