Group balances broken on 1 to 1 expenses

[krokosik]

When adding a group expense, I noticed some weird behaviour where the balances are reset to 0 if the expense involves only 1 person owing another different person.

How to replicate

1. Seed the database and login as Alice
2. In the Holiday Trip group create an expense where the full amount is paid by Bob and Alice is the only debtor
3. The balance is not reflected in our group view

What is more, I checked with pgAdmin that if it happens with an existing balance, it is reset to zero! This is a critical issue as it can really mess up a group and all expenses would have to be added again.

I looked in the code and saw a function called updateGroupExpenseForIfBalanceIsZero in splitService.ts. Its purpose is unclear to me, as it seems to set the group balances to 0 if the total balance with our friend is zero. This is quite odd to me, as groups should be independent of what happens outside off them, but maybe @KMKoushik could you tell me more about the reasoning behind it?

Nevertheless, it has a bug and I am submitting a fix, which I believe should be released ASAP.