allowing logicGroup branch scale to be zero

src/main/java/gov/usgs/earthquake/nshmp/tree/GroupBranch.java

 package gov.usgs.earthquake.nshmp.tree;
 
 import static com.google.common.base.Preconditions.checkNotNull;
-import static gov.usgs.earthquake.nshmp.data.DoubleData.checkIsPositiveAndReal;
+import static gov.usgs.earthquake.nshmp.data.DoubleData.checkIsPositiveAndRealOrZero;
 import static gov.usgs.earthquake.nshmp.tree.RegularBranch.checkId;
 
 /**
@@ -18,7 +18,7 @@ class GroupBranch<T> implements Branch<T> {
   GroupBranch(String id, T value, double scale) {
     this.id = checkId(id);
     this.value = checkNotNull(value);
-    this.scale = checkIsPositiveAndReal(scale);
+    this.scale = checkIsPositiveAndRealOrZero(scale);
   }
 
   @Override

src/main/java/gov/usgs/earthquake/nshmp/tree/LogicGroup.java

@@ -15,7 +15,8 @@ import java.util.List;
  * implements {@code LogicTree}, its sampling methods are overriden to throw an
  * {@code UnsupportedOperationException}. A logic group branch <i>may</i>
  * include a scale factor that is used in lieu of a branch weight; in most
- * real-world cases, the scale value is one.
+ * real-world cases, the scale value is one. The scale factor may also be zero
+ * to represent {@code null} branches.
  *
  * @param <T> the type of value stored in a branch
  * @author U.S. Geological Survey
@@ -97,7 +98,7 @@ public final class LogicGroup<T> extends AbstractList<Branch<T>> implements Logi
      *
      * @param id of the branch
      * @param value of the branch
-     * @param scale factor to apply in lieu of a branch weight
+     * @param scale factor to apply in lieu of a branch weight, may be zero
      * @return this {@code Builder} object
      * @throws NullPointerException if {@code id} or {@code value} is null
      * @throws IllegalArgumentException if {@code id} is empty or whitespace