RFR: JDK-8267394: Do not rely on object identity for empty valid Content instance

[Jonathan Gibbons]

On Thu, 20 May 2021 14:31:33 GMT, Hannes Wallnöfer <hannesw at openjdk.org> wrote:

> This is a simple cleanup to replace the sentinel `HtmlTree.EMPTY` text constant with an instance that achieves the same by overriding `isValid()`. I think this is the nicer solution, and it allows us to remove the special case identity check in `HtmlTree.add(Content)`.

This is questionable and probably wrong.

The use of the identity check was specifically to differentiate between a sentinel value (EMPTY) and values which just happen to be empty.

This will require detailed examination.

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Changes requested by jjg (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4130