Import of genesis backups fails

[shjortConcordium]

Bug Description
We have seen some genesis backups be rejected, because it is "incompatible with existing data". (Even against a fresh wallet)
The cause (and scope) of this should be determined and fixed.

Versions
Desktop wallet version 1.3.1

[orhoj]

This happens because the genesis exports do not contain the signatureThreshold field. When we try to insert the accounts into the database we get a constraint violation as we use an option:

useNullAsDefault: true

that means that the missing field is converted to an attempt to insert a NULL value, and we do not allow for NULL values for that field.

There are two paths:

1. Stop using useNullAsDefault which will stop converting undefined values to NULL insertions (proper solution, as there may be other places with similar bugs that we have not encountered yet. But the impact of the change has to be analyzed and not be put in a small hotfix).
2. Explicitly default signatureThreshold to 1 if missing in an import. The value will be updated with whatever is on-chain as soon as the user opens the account view anyway. (Quick fix to the genesis import issue).