This PR fixes a bug in the park polygon update code introduced in #1160. Because park polygons at zoom 4 are dissolved, the centroid is not present in the table at that layer. Thus, a separate update trigger is needed to avoid errors. This PR adds the separate trigger for the zoom 4 park layer. This PR likely resolves an unidentified blocker for #1245
park
Docs
Read the layer documentation at http://openmaptiles.org/schema#park

