provider/postgis: gracefully handle empty geometry collection

Added back geometry collection length check to ignore
0 length geometry collections. Added associated test case.

closes #429

provider/postgis/postgis.go

@@ -581,14 +581,8 @@ func (p Provider) TileFeatures(ctx context.Context, layer string, tile provider.
 			continue
 		}
 
-		// skip row if geometry collection empty.
-		g, ok := geometry.(geom.Collection)
-		if ok && len(g.Geometries()) == 0 {
-			continue
-		}
-
 		// decode our WKB
-		geom, err := wkb.DecodeBytes(geobytes)
+		geometry, err := wkb.DecodeBytes(geobytes)
 		if err != nil {
 			switch err.(type) {
 			case wkb.ErrUnknownGeometryType:
@@ -598,9 +592,15 @@ func (p Provider) TileFeatures(ctx context.Context, layer string, tile provider.
 			}
 		}
 
+		// skip row if geometry collection empty.
+		g, ok := geometry.(geom.Collection)
+		if ok && len(g.Geometries()) == 0 {
+			continue
+		}
+
 		feature := provider.Feature{
 			ID:       gid,
-			Geometry: geom,
+			Geometry: geometry,
 			SRID:     plyr.SRID(),
 			Tags:     tags,
 		}

provider/postgis/postgis_test.go

@@ -458,6 +458,18 @@ func TestTileFeatures(t *testing.T) {
 				LayerName:     "missing_geom_field_name",
 			},
 		},
+		"gracefully handle empty geometry collection": {
+			layerConfig: map[string]interface{}{
+				postgis.ConfigKeyLayerName: "empty_geometry_collection",
+				postgis.ConfigKeyGeomField: "geom",
+				postgis.ConfigKeyGeomType:  "polygon", // bypass the geometry type sniff on init
+				postgis.ConfigKeySQL:       "SELECT ST_AsBinary(ST_GeomFromText('GEOMETRYCOLLECTION EMPTY')) AS geom, !BBOX! AS bbox",
+			},
+			tile: slippy.NewTile(16, 11241, 26168, 64, tegola.WebMercator),
+			// the test query returns a single geometry but it's ignored so our cout is 0
+			expectedFeatureCount: 0,
+			expectedTags:         []string{},
+		},
 	}
 
 	for name, tc := range tests {