RFR: JDK-8322874: Redirection loop in index.html

[Jonathan Gibbons]

Please review a simple fix to address an apparently long-standing issue of generating a redirection loop in the top-level `index.html` file.

The fix is simply to remove some obsolete/unnecessary code in `HtmlConfiguration.setTopFile` that silently and incorrectly short-circuited the functionality of that method.

In addition to the fix, an `assert` statement is added in `IndexRedirectWriter` to ensure that invalid arguments are not provided. (An alternative would be explicit `if` statements than can throw NPE or ILA.

The test case is when there are no classes or interfaces specified in the command-line arguments.

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Commit messages:
 - JDK-8322874: Redirection loop in index.html

Changes: https://git.openjdk.org/jdk/pull/17270/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17270&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322874
  Stats: 83 lines in 3 files changed: 71 ins; 12 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17270.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17270/head:pull/17270

PR: https://git.openjdk.org/jdk/pull/17270