Fix panic when listing malformed SSO users in web UI

[hugoShaka]

This PR fixes a panic that happens if a user is of type SSO but doesn't have a CreatedBy.Connector field.

GetUserType can return sso even if CreatedBy.Connector is nil:

// GetUserType indicates if the User was created by an SSO Provider or locally.
func (u UserV2) GetUserType() UserType {
	if u.GetCreatedBy().Connector != nil ||
		len(u.GetOIDCIdentities()) > 0 ||
		len(u.GetGithubIdentities()) > 0 ||
		len(u.GetSAMLIdentities()) > 0 {

		return UserTypeSSO
	}

	return UserTypeLocal
}

Changelog: fix a bug causing a malformed user to break Teleport web UI's "Users" page

[backport-bot-workflows]

@hugoShaka See the table below for backport results.

Branch	Result
branch/v15	Create PR
branch/v16	Create PR
branch/v17	Create PR