Fix handling of "off" in encryption_enabled_by_default_for_room_type (#7822)

Fixes https://github.com/matrix-org/synapse/issues/7821, introduced in https://github.com/matrix-org/synapse/pull/7639

Turns out PyYAML translates `off` into a `False` boolean if it's
unquoted (see https://stackoverflow.com/questions/36463531/pyyaml-automatically-converting-certain-keys-to-boolean-values),
which seems to be a liberal interpretation of this bit of the YAML spec: https://yaml.org/spec/1.1/current.html#id864510

An alternative fix would be to implement the solution mentioned in the
SO post linked above, but I'm aware it might break existing setups
(which might use these values in the configuration file) so it's
probably better just to add an extra check for this one. We should be
aware that this is a thing for the next times we do that though.

I didn't find any other occurrence of this bug elsewhere in the
codebase.