fdo#73596 [DOCX] Auto Columns in Index

When number of columns in the Index is not specified
by the "\c" switch, then avoid adding the section breaks
explicitly before and after the Index Section and "\c '0'"
in the field identifier string.
Also added UT for the same.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/8104

Change-Id: I56826f72beea580d0ac5aef44d4a71fd29b77316