RFR: JDK-8256208: Javadoc's generated overview does not show classes of unnamed package

[Hannes Wallnöfer]

This is a simple fix to include the unnamed package when javadoc generates documentation for both unnamed and named packages. As elsewhere in javadoc, the package is listed as "Unnamed Package" (subject to localization).

Also, when documenting only code in the unnamed package, the index.html page redirects to the overview page of the unnamed package instead of the first documented class, which is generally more adequate for an overview page.

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Commit messages:
 - JDK-8256208: Javadoc's generated overview does not show classes of unnamed package

Changes: https://git.openjdk.java.net/jdk/pull/5911/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5911&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256208
  Stats: 55 lines in 5 files changed: 36 ins; 12 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5911.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5911/head:pull/5911

PR: https://git.openjdk.java.net/jdk/pull/5911