Bug 29077: Remove blank indexes before convert

Bug 29077: Remove blank indexes before convert

To test:
1 - Be using Elasticsearch
2 - Search on OPAC with dropdown set to 'Library catalog'
3 - Warns in plack-opac-error.log
4 - Apply patch
5 - Repeat
6 - No warns
7 - Search returns correct results

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>