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

services/users/errors.go

@@ -10,6 +10,7 @@ var (
 	VerifyUserError           = errors.New("could not update verify email field")
 	UserCountError            = errors.New("could not get the total number of the users.")
 	UserCreationDisabledError = errors.New("user creation is not allowed")
+	UserStoreIsEmptyError     = errors.New("could not check if the user store is empty")
 
 	GetTokenError            = errors.New("could not get token")
 	GetSessionError          = errors.New("could not get session")

services/users/users.go

@@ -13,6 +13,8 @@ import (
 	"github.com/mattermost/mattermost-server/v6/shared/mfa"
 	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 	"github.com/mattermost/mattermost-server/v6/store"
+
+	"github.com/pkg/errors"
 )
 
 type UserCreateOptions struct {
@@ -41,11 +43,9 @@ func (us *UserService) CreateUser(user *model.User, opts UserCreateOptions) (*mo
 
 	// Below is a special case where the first user in the entire
 	// system is granted the system_admin role
-	count, err := us.store.Count(model.UserCountOptions{IncludeDeleted: true})
-	if err != nil {
-		return nil, UserCountError
-	}
-	if count <= 0 {
+	if ok, err := us.store.IsEmpty(); err != nil {
+		return nil, errors.Wrap(UserStoreIsEmptyError, err.Error())
+	} else if ok {
 		user.Roles = model.SystemAdminRoleId + " " + model.SystemUserRoleId
 	}