SoftIndexFileStore process returns nothing when fetchValues is false

The soft index store process method relies on the value being set to determine if an entry was removed or not. Unfortunately when fetchValues is set to false this is always unset, which means it will return nothing.

The reason is because of the not null check on this line https://github.com/infinispan/infinispan/blob/e5958e36e3358b52095798044c083b65f021eba4/persistence/soft-index/src/main/java/org/infinispan/persistence/sifs/SoftIndexFileStore.java#L519