Append operations on non-existent keys don't increment cmd_set statistic

As a result of MB-28850, append operations did not increment the cmd_set statistic.

It seems the fix for this issue is incomplete as it will only increment the cmd_set statistic if the append is successful (due to the statistic being incremented in the 'success' state of the state machine).
This means that if the key does not exist, the statistic will not be incremented.

This is not a major issue, but it is inconsistent behavior compared to all of the other operations which will still increment their relevant statistics even if the key doesn't exist (e.g. GET, ADD).