Do not set geometry by default

A feature can have a geometry whose value is an ol.geom.Geometry instance or null.  A feature can also have no geometry property.  By default, a feature has no geometry property.  To set a geometry, one can be passed to the constructor (including null) or passed to setGeometry().
Merge pull request #2098 from tschaub/empty-feature

Do not set geometry by default.