Block: Avoid NPE in hasTransactions()

Field is nullable;

```java
@Nullable List<Transaction> transactions;
```

core/src/main/java/org/bitcoinj/core/Block.java

@@ -1003,7 +1003,7 @@ public class Block extends Message {
      * purely a header).
      */
     public boolean hasTransactions() {
-        return !this.transactions.isEmpty();
+        return (this.transactions != null) && !this.transactions.isEmpty();
     }
 
     /**