If you leave the field empty, you can't acces to your workspaces.
Here is the error you get

'''Caused by: org.ametys.plugins.repository.AmetysRepositoryException: An error occured executing the JCR query : //element(, ametys:site)
at org.ametys.plugins.repository.AmetysObjectResolver.query(AmetysObjectResolver.java:646)
at org.ametys.web.repository.site.SiteManager._getSite(SiteManager.java:481)
at org.ametys.web.repository.site.SiteManager.hasSite(SiteManager.java:410)
at org.ametys.plugins.workspaces.project.ProjectManager.getProjectsForSite(ProjectManager.java:812)
at org.ametys.plugins.workspaces.signup.UserSignupManager._getSignupSiteName(UserSignupManager.java:101)
at org.ametys.plugins.workspaces.signup.UserSignupManager.getSignupPages(UserSignupManager.java:82)
at org.ametys.web.usermanagement.UserSignupManager.getSignupPage(UserSignupManager.java:204)
at org.ametys.plugins.workspaces.signup.UserSignupManager.getSignupPage(UserSignupManager.java:76)
at org.ametys.web.frontoffice.FrontLoginScreenRedirectAction.act(FrontLoginScreenRedirectAction.java:128)'''

Configuration>espace projet>site catalogue