8171382: java.time.Duration missing isPositive method

Reviewed-by: rriggs, joehw, iris, bpb, scolebourne

src/java.base/share/classes/java/time/Duration.java

@@ -582,6 +582,20 @@ public final class Duration
     }
 
     //-----------------------------------------------------------------------
+    /**
+     * Checks if this duration is positive, excluding zero.
+     * <p>
+     * A {@code Duration} represents a directed distance between two points on
+     * the time-line and can therefore be positive, zero or negative.
+     * This method checks whether the length is greater than zero.
+     *
+     * @return true if this duration has a total length greater than zero
+     * @since 18
+     */
+    public boolean isPositive() {
+        return (seconds | nanos) > 0;
+    }
+
     /**
      * Checks if this duration is zero length.
      * <p>