calcSynthYield: Skip yield distribution when the layer 1 pool has zero BalanceAsset
[Aimed for V99]
This was previously part of !2563 (closed), now split off into its own specific MR.
This is a fairly minor MR: I saw that there was an explicit "if bucket is empty, skip it" check at the beginning of calcSynthYield
,
and thought it appropriate to do a similar explicit check for the layer 1 pool at the same time.