fdo#69649 RTF import: don't crash on empty TOC result

Regression from commit 9679e9c23216decb5f9f25f85b04cb3f25211111
(fdo#69649 Fix for pre-rendered Table Of Contents, 2013-12-30), the
situation that the field doesn't have a result wasn't handled, resulting
in a m_aTextAppendStack.pop() call in dmapper, without a matching
.push() one.

Change-Id: I456d9096d01fb27832a40f6b72802ec555d62647