Show generic error when fetching site-url/wp-json namespaces is empty during site credentials login

[JorgeMucientes]

Currently, if site namespaces are hidden due to some hosting provider configuration or security plugin, if the user attempt to log into the app they'll see the following error even for WooCommerce sites:

The error is misleading. The proposal is to update this piece of code from SiteWPAPIRestClient.kt:

                    hasWooCommerce = response?.namespaces?.any {
                        it.startsWith(WOO_API_NAMESPACE_PREFIX)
                    } ?: false

and simply return an error if response?.namespaces is empty.

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.