@Override
    public void read(FieldBase field, TensorFieldValue value) {
        throwOnlyJsonSupportedException(field, "TENSOR");
    }
Optional test and set condition. Common for document/update/remove elements
This variable is either set in VespaXMLFieldReader#read (reader for document)
or in VespaXMLUpdateReader#read (reader for update).
document/src/main/java/com/yahoo/vespaxmlparser/VespaXMLFieldReader.java