Don't look for signing key when sending to pwd recipients only

[rrrooommmaaa]

As we ignore the sign option in pwd-encrypted messages, then, in case all recipients are pwd recipients, we can make a shortcut and not search for a signing key, or at least disable this error message: Could not find account ${singleFamilyKeys.family} key usable for signing this encrypted message

[tomholub]

I would eventually like to verify signatures on FES. Since the ingredients are there: a signed message + ability to look up public keys.

[sosnovsky]

Password-protected messages are signed and verified on FES now - https://github.com/FlowCrypt/enterprise-server/pull/5158