Added "empty" constant to DoubleRange and FloatRange for uniformity.

runtime/src/jet/DoubleRange.java

@@ -23,6 +23,8 @@ public final class DoubleRange implements Range<Double> {
     private final double start;
     private final double size;
 
+    public static final DoubleRange empty = new DoubleRange(0, 0);
+
     public DoubleRange(double startValue, double size) {
         this.start = startValue;
         this.size = size;

runtime/src/jet/FloatRange.java

@@ -23,6 +23,8 @@ public final class FloatRange implements Range<Float> {
     private final float start;
     private final float size;
 
+    public static final FloatRange empty = new FloatRange(0, 0);
+
     public FloatRange(float startValue, float size) {
         this.start = startValue;
         this.size = size;