JaroWinklerDistance returns 0f when s1.length()=s2.length()=0 [LUCENE-2827]

StringDistance sd = new JaroWinklerDistance();
System.out.println(sd.getDistance("",""));

console prints 0.

but when use LevensteinDistance or NGramDistance, that console prints 1.0

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Migrated from [LUCENE-2827](https://issues.apache.org/jira/browse/LUCENE-2827) by tom liu
Environment:
```
JDK1.6.17
```

Start by reproducing JaroWinklerDistance.getDistance("", "") and compare its result with LevensteinDistance and NGramDistance, as shown in the issue. Trace the Jaro-Winkler implementation and add a regression test for two empty strings; done means it returns 1.0 consistently with the other distance implementations.

Written by the indexing model from the issue text.