fix crash for fdo#58306 when appending to OUStringBuffer initialised with ""
fix crash for fdo#58306 when appending to OUStringBuffer initialised with ""

don't call rtl_uString_newFromLiteral when literal has zero length as
that cheats and doesn't assign a buffer. what we really want to do
is assign a buffer with initial capacity ( but no content )

Change-Id: I9d642c453dee747aca6365abb15408c05f86bb52

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