fix linked data ref for geometry objects with varying number of members (#960)

pygeoapi/linked_data.py

@@ -326,6 +326,11 @@ def geom2schemageo(geom):
             except NotImplementedError:
                 poly_geom.extend(p.exterior.coords[:])
 
-    _ = [f'{x},{y}' for (x, y) in poly_geom]
-    f['schema:polygon'] = ' '.join(_)
+    try:
+        schema_polygon = [f'{x},{y}' for (x, y) in poly_geom]
+    except ValueError:
+        schema_polygon = [f'{x},{y},{z}' for (x, y, z) in poly_geom]
+
+    f['schema:polygon'] = ' '.join(schema_polygon)
+
     return f