Fix bug 5040:

Fix bug 5040:

* src/support/lstrings.{cpp,h}:
	- new optional param "bool keepemtpy" in getVectorFromString.
	  By default, empty content between two delimiters was/is not added to the vector.
	  This can be changed with this bool.

* src/insets/InsetIndex.cpp:
	- care for the case when plaintext returns nothing (e.g when ERT is used)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@25616 a592a061-630c-0410-9148-cb99ea01b6c8