Do not store empty Garrison tag in map xml files after rP23529

Differential Revision: https://code.wildfiregames.com/D2690
Patch by: Freagarach
Reported on forum: https://wildfiregames.com/forum/index.php?/topic/27867-tag-in-the-map-xml-file/

rP23529 introduced Garrison tag to the xml map files.
In case GarrisonHolder was present, but held no entities, empty tag was written. This is adding check for emptyness so that behaviour is prevented.