RTFDocumentImpl::resolveChars: don't send empty text tokens to dmapper
RTFDocumentImpl::resolveChars: don't send empty text tokens to dmapper

This speeds up importing unicode text a lot. Results of a sample of
chinese text of 5 pages:

- before: 18.351s
- after: 6.692s

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core