RTF_MR: ignore if collected text is empty
RTF_MR: ignore if collected text is empty

Sending an empty run would result in adding a space to the formula,
leading to incorrect result.

Change-Id: I1a01493cead726f667db0065deb4c145cba4ffa7

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