Null value dereference [LUCENE-6602]

In the file `BoostQueryNode.java`, `getChild` could be null and it is being dereferenced in the function `toString()`.

```Java
public QueryNode getChild() {
List children = getChildren();

if (children == null || children.size() == 0) {
return null;
}

return children.get(0);

}
```

```Java
public String toString() {
return "" + "\n"
+ getChild().toString() + "\n";
}
```

Should we not check if getChild is valid?

```Java
String s = (getChild() != null) ? getChild().toString() : "null";
return "" + "\n"
+ s + "\n";
```

---
Migrated from [LUCENE-6602](https://issues.apache.org/jira/browse/LUCENE-6602) by Rishabh Patel, updated May 09 2016

Open BoostQueryNode.java and inspect getChild() alongside toString(), focusing on the null-child path described in the issue. Verify the behavior for a node with no child, then confirm that toString() no longer dereferences null and produces the intended output.

Written by the indexing model from the issue text.