NPE in MultiFieldQueryParser [LUCENE-4777]

The method getRegexpQuery() in MultiFieldQueryParser is not overridden.
That leads to RegexpQueries where field=null (see example below).

```Java
QueryParser qp = new MultiFieldQueryParser(Version.LUCENE_41, new String[]{ "content" }, new StandardAnalyzer(Version.LUCENE_41))
Query q = qp.parse("path:foo/bar/foo"); // throws no ParseException
q.toString() // throws NPE
```

Workaround for MultiFieldQueryParser:

```Java
`@Override`
protected Query getRegexpQuery(String field, String termStr) throws ParseException {
if (field == null) {
List clauses = new ArrayList();
for (int i = 0; i < fields.length; i++) {
clauses.add(new BooleanClause(getRegexpQuery(fields[i], termStr), BooleanClause.Occur.SHOULD));
}
return getBooleanQuery(clauses, true);
}
return super.getRegexpQuery(field, termStr);
}
```

---
Migrated from [LUCENE-4777](https://issues.apache.org/jira/browse/LUCENE-4777) by André

Start at MultiFieldQueryParser and its getRegexpQuery entry point, then reproduce the provided path:foo/bar/foo example. The fix is done when parsing a regexp query across the configured fields produces a query whose toString() no longer throws a NullPointerException.

Written by the indexing model from the issue text.