Spurious multipleReferencesAllowed warning when serializing empty arrays

Empty arrays (FSArray) in UIMA are basically immutable. It is not possible to resize an array and also there are no elements within the empty array which can be replaced.

The deserialization code for some formats makes use of this fact and if it encounters an empty array, it just creates one empty array instance and then references that from all feature structures which need an empty array as their value.

When serializing such a CAS later, however, we get warnings if multipleReferencesAllow=true is not set on these features referencing the shared empty array.

The serialization code producing that warning needs to be changed to not issue a warning if the array in question is empty - because sharing then is absolutely ok since the array is immutable and empty.