buffer on small polygons is returning null

Buffering very small polygons (which a user obtained by buffering a Point by a small amount) returns null. For example,

    OGCGeometry p = OGCGeometry.fromText(
        "POLYGON ((177.0 64.0, 177.0000000001 64.0, 177.0000000001 64.0000000001, 177.0 64.0000000001, 177.0 64.0))"
    );
    OGCGeometry buffered = p.buffer(0.01);
    assertIsNull(buffered);

During the simplification step, the CrackAndCluster._do() collapses nearby points, which includes these nearby vertices. Instead of collapsing it to a Point, it collapses it to a Polygon with no rings, which eventually bubbles up as a null.

This, of course, causes an NPE when used.

cc @mbasmanova

[stolstov]

@jagill Buffer expects topologically simple input and should produce a topologicaly simple output. When a point is buffered with 1e-9 distance, the result polygon is smaller than the tolerance, thus it is considered non-simple, and is deleted.

[jagill]

Verified fix.