ui_info section missing in metadata.xml

This issue has been migrated from [#11405](https://github.com/matrix-org/synapse/issues/11405).

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### Description
I believe there is a bug when generating the `metadata.xml`
I think the example configuration might be very *very* wrong (for example, the contact person email address key has a typo causing it to break silently) and I can't figure out what else is wrong in there.

### Steps to reproduce

- define something in `saml2_config.sp_config.ui_info` in your `homeserver.yml` as shown in [the example config](https://github.com/matrix-org/synapse/blob/develop/docs/sample_config.yaml#L1638)
- see `` is not existent in `/_matrix/saml2/metadata.xml`

### Version information

- **Version**:
synapse v1.47.0
pysaml2 7.1.0

- **Install method**:
git clone

- **Platform**:
Debian 11 Linux Container on Proxmox7 (unprivileged, nesting)