Define geometries of placeholder features

[clarisma]

• A placeholder Way has no nodes (should have 2 zero nodes to make processing consistent)
• All other geometry-related methods return an X-coordinates of Integer.MIN_VALUE to indicate that the feature’s location is invalid/unknown (should invalid coordinates be 0/0 instead?)

[clarisma]

According to JTS:

• LINESTRING(0 0, 0,0) is not valid.
• EMPTY LINESTRING is valid.