Don't include empty boxes in their union.

The new behaviour matches what Rect does.

src/box2d.rs

@@ -213,8 +213,18 @@ where
         }
     }
 
+    /// Computes the union of two boxes.
+    ///
+    /// If either of the boxes is empty, the other one is returned.
     #[inline]
     pub fn union(&self, other: &Self) -> Self {
+        if other.is_empty() {
+            return *self;
+        }
+        if self.is_empty() {
+            return *other;
+        }
+
         Box2D {
             min: point2(min(self.min.x, other.min.x), min(self.min.y, other.min.y)),
             max: point2(max(self.max.x, other.max.x), max(self.max.y, other.max.y)),

src/box3d.rs

@@ -183,9 +183,18 @@ where
         Box3D::new(intersection_min, intersection_max)
     }
 
-    /// Returns the smallest box containing both of the provided boxes.
+    /// Computes the union of two boxes.
+    ///
+    /// If either of the boxes is empty, the other one is returned.
     #[inline]
     pub fn union(&self, other: &Self) -> Self {
+        if other.is_empty() {
+            return *self;
+        }
+        if self.is_empty() {
+            return *other;
+        }
+
         Box3D::new(
             Point3D::new(
                 min(self.min.x, other.min.x),

src/rect.rs

@@ -368,13 +368,6 @@ where
 {
     #[inline]
     pub fn union(&self, other: &Self) -> Self {
-        if self.size == Zero::zero() {
-            return *other;
-        }
-        if other.size == Zero::zero() {
-            return *self;
-        }
-
         self.to_box2d().union(&other.to_box2d()).to_rect()
     }
 }