Prevent NPE if all docs for the field are pruned during merge

During segment merges FieldConsumers for all fields from the source
segments are created. Yet, if all documents that have a value in a
certain field  are deleted / pruned during the merge the FieldConsumer
will not be called at all causing the internally used FST Builder to
return `null` from it's `build()` method. This means we can't store
it and run potentially into a NPE. This commit adds handling for
this corner case both during merge and during suggest phase since
we also don't have a Lookup instance for this segments field.

Closes #3555