Incorrect behavior for Pattern::split

## How to reproduce

```
jshell> com.google.re2j.Pattern.compile("x*").split("foo")
$1 ==> String[4] { "", "f", "o", "o" }

jshell> java.util.regex.Pattern.compile("x*").split("foo")
$2 ==> String[3] { "f", "o", "o" }
```

## Versions
- `java 11.0.10`
- `re2j 1.5`

Should I try for a PR?
