ApiQueryMapData: Fix PHP fatal when ParserOutput has no map data
ApiQueryMapData: Fix PHP fatal when ParserOutput has no map data

Follow-up to 52e679a797b8e.

Bug: T231561
Change-Id: I52e7a386778d21c3a91a8b20ea497384f5256625

wikimedia/mediawiki-extensions-Kartographer,wikimedia/mediawiki-extensions-Kartographer,wikimedia/mediawiki-extensions-Kartographer