Fix a bug introduced in Synapse v1.84.0 where workers do not start up when no `instance_map` was provided. (#15672)

* Fix #15669: always populate instance map even if it was empty

* Fix some tests

* Fix more tests

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

* CI fix: don't forget to update apt repository sources before installing olddeps deps

* Add test testing the backwards compatibility

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Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>