Fix crash when userinfo_endpoint is not set
Fix crash when userinfo_endpoint is not set

Not all OpenID Connect providers supply the userinfo endpoint (e.g. (dex)[https://github.com/coreos/dex/]). This is according to (spec)[https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata]:

userinfo_endpoint
    RECOMMENDED. URL of the OP's UserInfo Endpoint [OpenID.Core]. This URL MUST use the https scheme and MAY contain port, path, and query parameter components.
pingidentity/lua-resty-openidc