A NullPointerException is thrown in StreamingRenderer when a MapLayer contains no features

The problem occurs in GeoTools 2.3.1 (but is also present in the trunk), when rendering a map context that has an empty layer.
In this situation a NullPointerException is thrown in the StreamingRenderer class, in the processStylers(...) method, because
"geometryAttribute" variable is null, see code below:

final GeometryAttributeType geometryAttribute = schema.getDefaultGeometry();
final CoordinateReferenceSystem sourceCrs = geometryAttribute.getCoordinateSystem();

Attached you can find a testcase for replicating the problem and patch for the trunk version.