[DXEX-629] pages: fix error when dumping error_page without html property

[shushen]

✏️ Changes

The error_page object in tenant setting may contain only a url property for redirection and without a html property for templating. Previously we assume html is always present and thus will attempt to pass undefined as the data argument to fs.writeFileSync(), which now throws error on Node.js 14.

This PR fixes this by skipping writing the template file when error_page does not have the property html. Tests for url and show_log_link in error_page were also added.

🔗 References

• https://auth0team.atlassian.net/browse/DXEX-629
• Documentation on universal login error page: https://auth0.com/docs/universal-login/error-pages
• Management APIv2 explorer for tenant settings of error_page: https://auth0.com/docs/api/management/v2#!/Tenants/patch_settings

🎯 Testing

✅ This change has unit test coverage

🚫 This change has integration test coverage

🚫 This change has been tested for performance

[luisbritos]

Look good to me.