fixes https://github.com/spesmilo/electrum/issues/8837
Alternatively we could do a config upgrade (convert_version_*) to fix that specific issue,
but they are not safe against downgrade-upgrade-again, so probably not the best choice for
crash-inducing values.
And this kind of converter is a generic solution that can be useful for other configvars later
(though for most usages I expect we will also need a converter for the setter).
1d4e2ebed6