Is your feature request related to a problem? Please describe.

I use the MultiSamlStrategy Passport strategy. I'd like to provide metadata URLs to our IdPs, which I can build with MultiSamlStrategy#generateServiceProviderMetadata.

This works fine if I know the IdP's cert and can have getSamlOptions return an option object with cert. However, during initial setup, I may not immediately know the IdP's certificate. I should still be able to generate metadata for them, as (as far as I know) that metadata doesn't depend on their own certificate. However, if getSamlOptions returns cert: null (because I don't yet know it), the following assertion fails:

Describe the solution you'd like

Since the XML metadata doesn't depend on the IdP's cert, I'd like to be able to generate it before I do in fact know their cert.

Describe alternatives you've considered

When someone asks for our metadata URL, I can of course say "no, you give me your metadata URL first so I can get your cert". However, it would be very nice to be able to always provide it first.