fix(SAML): check on empty nameID (#8714)

# Which Problems Are Solved

If a SAML IdP did not send a `NameID` (even though required by the
specification), ZITADEL would crash.

# How the Problems Are Solved

- Check specifically if the `Subject` and its `NameID` is passed

# Additional Changes

None

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/8654

internal/idp/providers/saml/session.go

@@ -70,6 +70,10 @@ func (s *Session) FetchUser(ctx context.Context) (user idp.User, err error) {
 		return nil, zerrors.ThrowInvalidArgument(err, "SAML-nuo0vphhh9", "Errors.Intent.ResponseInvalid")
 	}
 
+	// nameID is required, but at least in ADFS it will not be sent unless explicitly configured
+	if s.Assertion.Subject == nil || s.Assertion.Subject.NameID == nil {
+		return nil, zerrors.ThrowInvalidArgument(err, "SAML-EFG32", "Errors.Intent.ResponseInvalid")
+	}
 	nameID := s.Assertion.Subject.NameID
 	userMapper := NewUser()
 	// use the nameID as default mapping id