Bug len(None) communication extension inline attachments

### Describe the bug

With the new version 1.11.0 and the introduction of inline attachments for the azure communication extension,
a bug raises with a `len of None object` error when not setting the inline attachments.

Thanks.

### Related command

az communication email send

### Errors

object of type 'NoneType' has no len()

### Issue script & Debug output

```
az communication email send \
--connection-string "${AZURE_COMMUNICATION_SERVICE_ENDPOINT}" \
--sender "DoNotReply@${AZURE_FROM_SENDER_DOMAIN}" \
--to "${to_email}" \
--subject "TEST" \
--text "See the attachment." \
--attachments ${filename} \
--attachment-types plain/text
```

### Expected behavior

Should work.

### Environment Summary

azure-cli 2.64
azure communication extension 1.11.0

### Additional context

Using `azure communication extension 1.10.0` everything work as expected..