* neomutt: Fix eval error when primary account not enabled
If neomutt is enabled for an account, but not the primary account, the
configuration will fail with "list index 0 is out of bounds".
This adds the first neomutt-enabled account as a fallback.
* neomutt: add regression test/update tests
The `accounts.email.accounts.<name>.neomutt.extraConfig` option is
included twice in the resulting config file for the account. One time as
part of the `mraSection`, one time as part of `accountStr` (`accountStr`
includes the `mraSection`). This removes that duplication. I opted to
keep the one in `accounStr`, since `extraConfig` doesn't necessarily
have anything to do with the `mraSection`.
