Changes TaggerFstCorpus to allow SolrDocuments that do not define the "storedField"

There are two possible reasons, why `IndexableField storedField = document.getField(storedFieldName);` returns null:

1. The field definition in the schema is faulty (`stored=false`) and therefor the labels for the current entity are missing in the Index
2. The field definition is correct, but the entity does not have any labels

If `storedFieldName` represents the field use to store labels in a specific language (e.g. German) it is a total valid case that an entity might not have a label in that specific language.

This change will allow to create per language FST corpora as suggested by [Issue #5](https://github.com/OpenSextant/SolrTextTagger/issues/5)