Fix bug causing empty product URL paths

[indiebytes]

Resolves issue #63:

If Divante\VsbridgeIndexerCatalog\Model::getProductUrlSuffix() returns null it will cause an issue in Divante\VsbridgeIndexerCatalog\Model::addUrlPath().

$rewrite = mb_substr($rewrite, 0, -strlen($urlSuffix));

strlen(null) will return 0 which in turn leads to mb_substr() returning an empty string which will be indexed as url_path for the product.

This pull request also contain a minor update that cast $this->productUrlSuffix to string once it's fetched from Magento config.