Replace db count query in user signup process (#18072)

store/store.go

@@ -424,6 +424,7 @@ type UserStore interface {
 	DeactivateGuests() ([]string, error)
 	AutocompleteUsersInChannel(teamID, channelID, term string, options *model.UserSearchOptions) (*model.UserAutocompleteInChannel, error)
 	GetKnownUsers(userID string) ([]string, error)
+	IsEmpty() (bool, error)
 }
 
 type BotStore interface {