When no experimental features were enabled (e.g. Matrix RTC and Matrix Authentication Service both disabled), the generated homeserver.yaml contained `experimental_features: null`, which Synapse v1.157.0+ does not tolerate and crashes on startup.

The `experimental_features` configuration is now constructed via a dedicated `matrix_synapse_experimental_features` variable (split into `_default`, `_auto` and `_custom`, as is customary), which always yields a mapping (potentially an empty one).

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/5455

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>