return null envelope when building from an empty geometry

Author: bosborn

src/main/java/mil/nga/sf/util/GeometryEnvelopeBuilder.java

@@ -45,6 +45,11 @@ public static GeometryEnvelope buildEnvelope(Geometry geometry) {
 
 		buildEnvelope(geometry, envelope);
 
+		if (envelope.getMinX() > envelope.getMaxX()
+				|| envelope.getMinY() > envelope.getMaxY()) {
+			envelope = null;
+		}
+
 		return envelope;
 	}