"Untitled Account" shown in Import Form if Transaction Preview has Nil UUID for source or destination

[morremeyer]

Describe the bug
When the Transaction Preview has a Nil UUID for either source or destination, that account is shown as "Untitled Account" instead of being unset

To Reproduce
Steps to reproduce the behavior:

1. Import transactions that do not get a destination/source account set in the Transaction Preview (this is the case when they are not matched by any MatchRule)
2. See that for all these transactions, source/destination is "Untitled Account"

Expected behavior
The Nil UUID is treated as if the value was unset/null.

Additional context

The Nil UUID is 00000000-0000-0000-0000-000000000000 and may be returned by the backend for any UUID field. While this does not occur in practice in most places, since we enforce a UUID to be set for most fields.

Ideally, the frontend checks for 00000000-0000-0000-0000-000000000000 in all ID fields and treats that as equivalent to null.