[Audit_RV] Inconsistent sign for 0 in SimpleImbalance

[virgil-serbanuta]

The SimpleBalance implementation does not use a consistent sign for 0. By default 0 is negative, and add also makes 0 negative, but sub makes it positive.

In the current implementation, SimpleBalance is used only with negative values, so sub should not produce a 0 value. Still, it may make sense to make the sign of 0 consistent, e.g. by making it always be negative.

https://github.com/galacticcouncil/HydraDX-node/blob/8de6222f967cc1bfe120207a775e8f0dc8eb92a5/pallets/omnipool/src/types.rs#L206