NPE in Flexible StandardQueryParser on boosting [LUCENE-5491]

The following exception
```
java.lang.IllegalArgumentException: field name should not be null!
at org.apache.lucene.queryparser.flexible.core.config.FieldConfig.(FieldConfig.java:36)
at org.apache.lucene.queryparser.flexible.core.config.QueryConfigHandler.getFieldConfig(QueryConfigHandler.java:59)
at org.apache.lucene.queryparser.flexible.standard.processors.BoostQueryNodeProcessor.postProcessNode(BoostQueryNodeProcessor.java:54)
at org.apache.lucene.queryparser.flexible.core.processors.QueryNodeProcessorImpl.processIteration(QueryNodeProcessorImpl.java:99)
at org.apache.lucene.queryparser.flexible.core.processors.QueryNodeProcessorImpl.processChildren(QueryNodeProcessorImpl.java:125)
at org.apache.lucene.queryparser.flexible.core.processors.QueryNodeProcessorImpl.processIteration(QueryNodeProcessorImpl.java:97)
at org.apache.lucene.queryparser.flexible.core.processors.QueryNodeProcessorImpl.process(QueryNodeProcessorImpl.java:90)
at org.apache.lucene.queryparser.flexible.core.processors.QueryNodeProcessorPipeline.process(QueryNodeProcessorPipeline.java:90)
at org.apache.lucene.queryparser.flexible.core.QueryParserHelper.parse(QueryParserHelper.java:255)
at org.apache.lucene.queryparser.flexible.standard.StandardQueryParser.parse(StandardQueryParser.java:168)
```

is caused by boosting a tokenizable phrase field within a group.

```java
public void testFail() throws Exception {
test("(mimeType:\"text-html\")");
}

public void testOkay() throws Exception {
test("mimeType:\"text-html\"");
}

static void test(String qs) throws Exception {
Analyzer sa = new StandardAnalyzer(Version.LUCENE_46);
StandardQueryParser qp = new StandardQueryParser(sa);
qp.getFieldsBoost().put("mimeType", 1f);
qp.parse(qs, "content");
}
```

---
Migrated from [LUCENE-5491](https://issues.apache.org/jira/browse/LUCENE-5491) by André , updated May 09 2016

Start with BoostQueryNodeProcessor.java and the QueryConfigHandler/FieldConfig stack frames, then run the provided testFail and testOkay cases using StandardQueryParser. The fix is complete when boosting the tokenizable mimeType phrase inside a group no longer throws the null-field exception and the existing ungrouped case still parses.

Written by the indexing model from the issue text.