xmerge: use String.length()==0 instead of String.equals(empty string)

Change-Id: I41b64c2dccad41193e15b6be2e62ce9dd63e0c3a
Reviewed-on: https://gerrit.libreoffice.org/11358
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>