Searching for something like (foo)|(bar) and replacing with $1$2 would fail
assertion in appendCopy in include/rtl/ustrbuf.hxx, because beginIndex is
negative (-1), because one of the references is always empty (SearchResult
at TextSearch::ReplaceBackReferences() has both startOffset and startOffset
equal to -1).

In this case, we should simply return an empty string.

Change-Id: Ibf91b1d17ab21c279cfcdc31e84d0c2eae567a53
Reviewed-on: https://gerrit.libreoffice.org/62248
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>