Fix config regression around falsy experimental_features (None)

[MadLittleMods]

Fix config regression around falsy experimental_features (None)

Examples:

experimental_features:

experimental_features: null

experimental_features: ~

Fix #19986

Regressed in #19539

The plan is to create a patch release and ship this in v1.157.1

Dev notes

SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.config.test_experimental

SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.config.test_server

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[MadLittleMods]

This matches what we already do:

synapse/synapse/config/experimental.py

Line 65 in 3622579

experimental = config.get("experimental_features") or {}

[MadLittleMods]

This is here so the CI runs but the plan is to remove it in favor of coalescing it with #19539

[reivilibre]

since it's going out in a point release, wouldn't it need its own bugfix entry?

[MadLittleMods]

That makes sense 👍

[MadLittleMods]

Thanks for the review @reivilibre 🦦