* 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
This resolves the error
The option `accounts.email.accounts.xyz.neomutt.sendMailCommand`
is defined both null and not null, in
`…/home-manager/modules/accounts/email.nix' and
`…/home-manager/modules/accounts/email.nix'.
that would occur previously when both neomutt and msmtp were enabled
for an account.
