Prevent AS user check if no AS registered (#392)

When a user registered on a homeserver with no application services
 registered, registration would check if the meta-regexp object matched
 the proposed user's new username.

 Apparently "" is a regex that matches everything, so every user was
 then barred from registering as they were supposedly registering inside
 an AS' exclusive namespace.

 This change prevents that check from happening by setting the exclusive
 regex to ^$ instead, preventing any matches from occurring.

 We also prevent the check for exclusivity if there are no namespaces
 registered for performance.

Signed-off-by: Andrew Morgan (https://amorgan.xyz) <andrew@amorgan.xyz>

src/github.com/matrix-org/dendrite/clientapi/routing/register.go

@@ -381,8 +381,9 @@ func Register(
 	}
 
 	// Make sure normal user isn't registering under an exclusive application
-	// service namespace
+	// service namespace. Skip this check if no app services are registered.
 	if r.Auth.Type != "m.login.application_service" &&
+		len(cfg.Derived.ApplicationServices) != 0 &&
 		cfg.Derived.ExclusiveApplicationServicesUsernameRegexp.MatchString(r.Username) {
 		return util.JSONResponse{
 			Code: 400,

src/github.com/matrix-org/dendrite/common/config/appservice.go

@@ -113,6 +113,12 @@ func setupRegexps(cfg *Dendrite) {
 	// regex and deny access if it isn't from an application service
 	exclusiveUsernames := strings.Join(exclusiveUsernameStrings, "|")
 
+	// If there are no exclusive username regexes, compile string so that it
+	// will not match any valid usernames
+	if exclusiveUsernames == "" {
+		exclusiveUsernames = "^$"
+	}
+
 	// TODO: Aliases and rooms. Needed?
 	//exclusiveAliases := strings.Join(exclusiveAliasStrings, "|")
 	//exclusiveRooms := strings.Join(exclusiveRoomStrings, "|")