An hst:site should load its configuration by convention if the hst:configurationpath is empty or missing

thus, if we have:

hst:configurations
myproject

hst:sites
myproject
myproject-preview

then if an empty hst:configurationpath is configured, both sites should get hst:configurations/myproject as config. If there would also be a hst:configurations/myproject-preview, then the preview site should get that one