Join: Use Precondition#checkNotNull everywhere

src/main/java/org/assertj/core/condition/Join.java

@@ -42,7 +42,7 @@ public abstract class Join<T> extends Condition<T> {
    */
   @SafeVarargs
   protected Join(Condition<? super T>... conditions) {
-    if (conditions == null) throw conditionsIsNull();
+    checkNotNull(conditions, conditionsIsNull());
     this.conditions = Arrays.stream(conditions).map(Join::notNull).collect(toList());
   }
 
@@ -53,14 +53,14 @@ protected Join(Condition<? super T>... conditions) {
    * @throws NullPointerException if any of the elements in the given iterable is {@code null}.
    */
   protected Join(Iterable<? extends Condition<? super T>> conditions) {
-    if (conditions == null) throw conditionsIsNull();
+    checkNotNull(conditions, conditionsIsNull());
     this.conditions = new ArrayList<>();
     for (Condition<? super T> condition : conditions)
       this.conditions.add(notNull(condition));
   }
 
-  private static NullPointerException conditionsIsNull() {
-    return new NullPointerException("The given conditions should not be null");
+  private static String conditionsIsNull() {
+    return "The given conditions should not be null";
   }
 
   private static <T> Condition<T> notNull(Condition<T> condition) {