If an expense is filtered to only be paid by certain people, and you remove all those people, no one is paying the expense

### Reproduction steps:
- Add a third roommate
- Filter an expense to only be paid by the third roommate
- Remove the third roommate

#### Expected behavior:
- UI doesn't let you
or:
- Expense is marked as invalid
or:
- Expense is automatically filtered so that everyone remaining shares it (probably not great)

#### Actual behavior:
- The expense now has no one paying it, a state that the UI otherwise doesn't allow you to enter

Start by reproducing the listed roommate and expense-filter sequence in the UI, then trace the code handling roommate removal and filtered expense payers. Compare the available expected behaviors and ensure the application cannot leave an expense with no paying people, with a regression test or equivalent verification for the reproduction case.

Written by the indexing model from the issue text.