fix: Avoid actions with debt/coll zero

[bingen]

Closes # 9

[danielattilasimon]

The change is okay but please don't close #9 with this! The issue explicitly says:

Refactor withdrawETHGainToTrove such that it doesn't use an internal function intended for borrowers.

This change does not address the core issue, which is:

It's somewhat hacky to use the internal function _adustTrove that's really intended for borrower operations

[bingen]

The issue explicitly says: ...

Yes, I know, but I just disagree. That’s the proposed solution to the problem of burning 0 LUSD. But I don’t see a big difference between withdrawing ETH from SP to a Trove or using addColl, which uses that internal _adjustTrove function. Making moveETHGainToTrove not to use it would be an unnecessary code duplication in my opinion.

[bingen]

Another thought I had while writing this PR was to just remove moveETHGainToTrove, and have it as a zapper. What do you think about that?

[danielattilasimon]

Another thought I had while writing this PR was to just remove moveETHGainToTrove, and have it as a zapper. What do you think about that?

Yes, that sounds better.